ThePunishedSnake: Basically only three new playable character, and we don't know if each will come with a short campaign or you can just use them in main campaign (but the second point make me afraid this will be the case). Other stuff like Turbo Mode and Legendary Dark Knight Mode was already in PC version of the game, missing on 360 and PS3.
Capcom should release this as DLC on PC, but since there's a separate app, this won't be the case.
Well its Capcom so what else can you expect, there is no new campaign as far as I know but there is atleast some new gameplay content unlike the other remasters. The most they will do is offer a discount to the previous owners of the game on PC but knowing Capcom that is highly doubtful, releasing it as DLC is probably out of the question for them.
Personally I haven't played DMC 4 before though so as soon as I got wind of this Special Edition I knew it was better to wait for the release and then be patient until it is significantly discounted. ;)