we were luck to of had a decent squal to Toy Story. Disney demanded a TS 2, but Pixar didnt like sequals at the time. but Disney basically said they would do a sequal without Pixar's involvment anyway (as they owned the rights, I think, and had total power over it). Pixar i guess didnt like the idea of Disney messing with their work, so they presented a TV 30 minute mini movie. Disney loved it and somehow wangled Pixar to make a longer feature out of it.
I do like the TS 2 - but i dont like the idea of how Disney forced Pixar to make it, even though millions have been made from it.
I am suprised further down the time line, a Cars sequal was granted. the movie didnt do all that great at the box office (in comparison to others) and wasnt the best Pixar film. But i believe the Merchandise franshise is one of the best ever in movie history - making over 1Billion$$$ in sales. i guess that was enough to push for a sequal!
the only Pixar films i would of liked to see sequals to are Monsters Inc (which i believe is being made) and The Incredibels.