NOTE: This review is 80% the same as my review on Marvel vs. Capcom 3.
This is the updated version of Marvel vs. Capcom 3, with extra 12 characters, certain gameplay changes, and etc. This is the definitive version of Marvel vs. Capcom 3 so far, so you might want to get this if you missed playing Marvel vs. Capcom 3 last year.
Storyline
Source: http://en.wikipedia.org/wiki/Marvel_vs._Capcom_3:_Fate_of_Two_Worlds#Plot
There, that's the whole story of the game. Besides that, after beating the final boss with a specific character, you will get to view his/her/it's ending. Some are good, some are not. The ending only explained using texts and comic illustrations without any voiceover, which dampens the ending's impact to players.
In Ultimate version, although you can view a brief prologue of the game on how the game started, the story is still very disappointing. The storyline is weak, and it is not well explained in the game, and most of the endings for each characters in the game are quite disappointing to me. Few of the characters endings are altered slightly. I don't know why they did it, but they did it anyway. Don't expect this game to have a great storyline if you are picking this up.
Gameplay
The game plays like previous MVC games, except with more simplified controls. There are 4 main attacking buttons. Light attack, medium attack, hard attack, and launcher attack. In this game, it is much easier pulling off Aerial Combos that can devastate your opponents.
As in many fighting games, the objective to win a match is to defeat all characters of your opponent.
You can play with three characters in your team, and you can choose either Marvel side, Capcom side, or both together. There is a lot of characters to be used in this game, as there are more than 30 characters to be used in the game, with some unlockable characters as well as two DLC characters currently.
Each characters have their own pros and cons, depending on player's point of views. Dante has a large arsenal of techniques to fight various types of opponents, but he has lower attack power. Chun-Li has great speed in movement and attacking, but most of her attacks are close ranged. The main idea to make a solid team in the game is to get characters that can offsets' another's weaknesses, like teaming up Iron Man with Chun-Li to offset her lack of ranged attacks, or Hulk with Phoenix to offset her very low HP.
Gamers have each of their own fighting styles to fight another gamer. Some might use a team of Iron Man, Akuma, and Ryu to dominate his opponents in pure Hyper Combo moves as well as projective attacks, while some uses a team of Dante, X-23, and Wolverine to attack his opponents up and close to prevent them from counterattacking. It all depends on you.
Each characters have their own special moves, like Ryu has Hadouken and Shoryuken as his special moves. Some of their special moves have advanced forms called Hyper Combos.
In Ultimate version, this game have extra 12 new characters. 6 for Capcom and 6 for Marvel. Some of them are pretty cool to use, such as Ghost Rider, Vergil, and Iron Fist. Personally, I have difficulty to use other characters such as Phoenix Wright as his techniques are quite hard to master.
The gameplay is refined with proper gameplay balancing such as stat changes, with the addition of 12 new characters. Although the gameplay is refined, that doesn't stop you from being defeated by other players due to lack of skill. Keep playing until you have become much better to fight other human players, like your friends.
This is the updated version of Marvel vs. Capcom 3, with extra 12 characters, certain gameplay changes, and etc. This is the definitive version of Marvel vs. Capcom 3 so far, so you might want to get this if you missed playing Marvel vs. Capcom 3 last year.
Storyline

Play as Capcom or Marvel characters, and defeat Galactus to save the world. Simple as that. But not all characters have good intentions after defeating Galactus, like Wesker.
I don't know if the game has a story, but apparently, there is one:
Doctor Doom has assembled the greatest villains of the Marvel Universe and has joined forces with Albert Wesker in order to unite their respective universes in an effort to conquer both. However, this course of action awakens a great and powerful threat that could potentially destroy both worlds. It is up to the heroes of the Marvel and Capcom universes to put a stop to this evil before it is too late
Source: http://en.wikipedia.org/wiki/Marvel_vs._Capcom_3:_Fate_of_Two_Worlds#Plot
There, that's the whole story of the game. Besides that, after beating the final boss with a specific character, you will get to view his/her/it's ending. Some are good, some are not. The ending only explained using texts and comic illustrations without any voiceover, which dampens the ending's impact to players.
In Ultimate version, although you can view a brief prologue of the game on how the game started, the story is still very disappointing. The storyline is weak, and it is not well explained in the game, and most of the endings for each characters in the game are quite disappointing to me. Few of the characters endings are altered slightly. I don't know why they did it, but they did it anyway. Don't expect this game to have a great storyline if you are picking this up.
Gameplay

Plays like previous Marvel vs. Capcom games, but with better controls. Make your team of pure Marvel characters, pure Capcom characters, or even a mix of them.
The game plays like previous MVC games, except with more simplified controls. There are 4 main attacking buttons. Light attack, medium attack, hard attack, and launcher attack. In this game, it is much easier pulling off Aerial Combos that can devastate your opponents.
As in many fighting games, the objective to win a match is to defeat all characters of your opponent.

Every characters have their own strengths and weaknesses. This game have slightly rebalanced gameplay with some stat changes on certain characters to avoid overpowering against other players.
You can play with three characters in your team, and you can choose either Marvel side, Capcom side, or both together. There is a lot of characters to be used in this game, as there are more than 30 characters to be used in the game, with some unlockable characters as well as two DLC characters currently.
Each characters have their own pros and cons, depending on player's point of views. Dante has a large arsenal of techniques to fight various types of opponents, but he has lower attack power. Chun-Li has great speed in movement and attacking, but most of her attacks are close ranged. The main idea to make a solid team in the game is to get characters that can offsets' another's weaknesses, like teaming up Iron Man with Chun-Li to offset her lack of ranged attacks, or Hulk with Phoenix to offset her very low HP.
In Ultimate version, the game now has slightly readjusted gameplay, with certain stat changes to most of characters available in Marvel vs. Capcom 3, such as reduced health of Sentinel, etc. This is to avoid overpowering your opponents easily in both offline and online modes. Some characters also have their moves toned down or buffed up, such as addition of new moves for Ryu. The gameplay changes are welcome, but still, experienced players can still beat you easily if they master the usage of their favourite characters despite some stats changes. Skill is still the primary element in playing fighting games against another player.

Play another 12 new characters in the game, such as Ghost Rider, Vergil, Nova, Strider Hiryu and Iron Fist
Gamers have each of their own fighting styles to fight another gamer. Some might use a team of Iron Man, Akuma, and Ryu to dominate his opponents in pure Hyper Combo moves as well as projective attacks, while some uses a team of Dante, X-23, and Wolverine to attack his opponents up and close to prevent them from counterattacking. It all depends on you.
Each characters have their own special moves, like Ryu has Hadouken and Shoryuken as his special moves. Some of their special moves have advanced forms called Hyper Combos.
In Ultimate version, this game have extra 12 new characters. 6 for Capcom and 6 for Marvel. Some of them are pretty cool to use, such as Ghost Rider, Vergil, and Iron Fist. Personally, I have difficulty to use other characters such as Phoenix Wright as his techniques are quite hard to master.

Hyper Combos are not only flashy, but are very devastating.
Each characters have their own Hyper Combo moves. Hyper Combo moves are advanced techniques that can damage your opponent's character a lot. Most characters have 3 Hyper Combo moves, like Dante has Hyper Combos of Million Dollars, Devil Trigger, and Devil Must Die. In order to do a Hyper Combo, you need to deplete your Hyper Gauge, which is filled by attacking your enemies, or getting hit by your enemies. All characters Hyper Combos that requires only 1 bar of Hyper Gauge, but some require more than 2. In order to build your Hyper Gauges, you need to attack your enemies. It can also be filled by being attacked by your enemies as well.
Crossover Combination are a combination of three Hyper Combos to defeat your enemies. If one of your characters are KO'ed, only 2 characters will carry out Crossover Combination. Although it is a good move that can depletes your opponent's characters health fast, it is not advisable to use it often as it depletes a lot of your Hyper Gauge.

Vergil is awesome in this game.
When you see your character has low health, you can switch out immediately with another character to prevent him/her from being KO'ed. This is vital as having KO'ed members in your team can put you in a disadvantage against others. Also, you can use Assists to help your character in assaulting your opponents. You can set up a different Assist move for each character, like Jam Session for Dante, Uni Beam for Iron Man, and Hadouken for Ryu. Setting up proper Assist moves for each characters will help out a lot in playing this game.
Blocking enemy's attacks is important to preserve your health. It is essential when your opponent uses a Hyper Combo or Crossover Combination, as both can deal huge damage, especially the latter.
There are few game modes in this game, like Arcade mode, Mission mode, and Training mode. Arcade mode requires you to fight several rounds to view an ending, Mission mode requires you to perform moves to proceed, and Training mode allows you to hone your gaming skills. However, the main draw in this game is the online matches, as you can played Ranked Matches to get higher rank. Be aware that playing with another human player is a different experience than playing CPU. All fighting games essentially like that, as humans are unpredictable in many ways.
There is also a new mode addition called Heroes and Heralds mode, where it allows you to take sides as the good guys or the bad guys. As the bad guys (Team Heralds), your characters will be coloured silver, like the avatars of Galactus that you fight in his stage. In this mode, you can equip your characters with various character cards for bonuses, such as increased health. The mode itself is some kind of campaign mode, where you choose a team of Heroes or Heralds and try to take control of an area. Total control of an area is 100%, and winning one match gives you a certain percentage of control. As you win more and more matches in an area, the difficulty will increase making it much harder for you to win. It is a fun mode for you to play. You can play Heroes and Heralds mode in either Online or Offline modes.
The gameplay is refined with proper gameplay balancing such as stat changes, with the addition of 12 new characters. Although the gameplay is refined, that doesn't stop you from being defeated by other players due to lack of skill. Keep playing until you have become much better to fight other human players, like your friends.