Many sequels simple cash in on the success of the first without ever changing. Crash Bandicoot 3, Tomb Raider 3, and Road Rash 3D are prime culprits. The Crash series could have evolved into something really great. That's not Core's fault so much as it is Eidos', who has forced a sequel each year. And Road Rash 3D, well, that was just abominable.
Sequels are numerous and take on a variety of forms.
Jesus, I love the Street Fighter series, honest to God, but Capcom has truly perfected the art of running in place. Sequels are proven money makers, and consoles need them. But with more mediocre sports, wrestling, fighting and action sequels on the way, the fact is, the PlayStation has become the wasteland of the sequel. Good sequels exist. These are fresh sequels at work, where the new installment is actually fundamentally better than the first.