The trailer wasn't panned, it was just a first draft of what Dante could look like. Reaction to it was lukewarm, so I guess Capcom decided to change it. Me, I thought there was something weird going on with his face; he didn't look like Dante. It had a Final Fantasy vibe, maybe slightly Vaan-like.
I don't know if there's another view of the coat Dante's got in that DMC4 promotional picture, but it's not exactly the DMC3 coat. The DMC3 coat collar has shorter straps, and the promo one doesn't seem to have any leather strap thingamabobs. Also one or two fewer zippers.
As for the final Dante coat design, I had no problems with it. The theme they were going with was partly 'western', and partly biker, I guess. So why so many people had such big problems with the chaps, no idea... those kinds of pants are pretty normal for bikers, nothing 'gay' about it. It's a bit more flamboyant because it's partly red, but if that's the problem people had with it, I find it strange they never criticize DMC1 Dante for wearing red. The DMC3 costume looks kind of boring in comparison to it, IMO. My favorite is still the DMC2 costume, though, followed by DMC1.