Yeah, same here. I was skeptical at first and it does feel like Pixar are pushing their luck with these upcoming squeals and prequels. Buuuuut, that was a pretty good teaser.
Problem is, we would have had all these sequels if Pixar and Disney split, and they'd all be completely terrible. I needn't remind anyone of the tragedy that was almost Toy Story 3. As Disney owned the characters, they could do whatever the heck they wanted with them, and Pixar couldn't do anything about it.
I think I read that Monsters Inc was supposed to be a timeskip sequel where they visit Boo as a teenager or something, and get lost in the human world, which could only lead to basically the same 2319 gag from the first movie, only in a different world. So basically the same movie in reverse. Somehow, someone had the brilliant notion that would have ruined the heartwarming ending and they did a prequel instead.
Now, Pixar has been batting pretty good as far as sequels go, the only sequel that people didn't like was a sequel to a movie that people were barely fine with in the first place (Cars). And while it wasn't Pixar's finest, it was nice for them to take a rest from the artistic pathos of their last few films and do something that's kid friendly. Their weakest films certainly hold up against the stronger films of other 3rd party CGI studios. I have a lot of faith in this film. I liked Brave and all, but it felt like it was missing something and it just feels like something completely alien to Pixar. This film seems to be a nice breather until the Dinosaur film, and the Journey into the Human Mind film that sounds so incredibly artistic in its premise it could just alienate anyone under the age of 20.