This is probably the best thing that could happen to Marvel and Disney. Remember people, Disney already has all the Marvel cartoons airing on their cartoon channel, and they've been getting really good ratings for those, so Disney's been involved with Marvel way before this deal was even in talks.
Marvel needs a powerful company financially backing them up like how Warner Bros. owns DC, and Disney profits from the comics/movies/games/toys. Disney will probably also increase production of Marvel products to place in their amusement parks and stores, meaning Marvel will have more exposure. Disney will also end up not alienating the young male demographic like they've been doing in recent years.
Also from what I've heard Disney wants to start pushing some of the lesser known Marvel characters like Ant-Man and Dr. Strange, and that's never a bad thing IMO. Once the movie rights for Spider-Man, the X-Men, Blade, Daredevil, and the Fantastic Four are up, I expect Disney and Marvel to re-claim them pretty quickly. At worst, the most you'll see Disney do with Marvel outside of what's been already mentioned is start making some Disney character comic books for the kiddies that are separate from the Marvel line. No big deal.