The more movies I see, the more it becomes apparent that the best sequels are for movies that were initially intended to have sequels. Obviously this doesn't apply for movies like Lord of the Rings or Harry Potter that are based on series of books, but it seems to hold very well for Xmen (good, original was written to support sequels), Men in Black (bad, sequel was decided on after the original was successful), Ace Ventura (once again, a horrible attempt to cash in on the surprising success of the first), and many other recent films.

Usually the problem is that the plot of the first movie doesn't lend itself to a continuation, but often the writers just pull a Sister Act and rehash the exact same story with the same characters.

Based on this postulate, we can form some theories about this summer's crop of sequels:

Terminator 3: Bad, although Arnold's hilarity may save this one
Legally Blonde 2: Bad
Dumb and Dumberer: A whole new level of bad, since the actor that made the first movie successful won't even be in it (a la Disney's straight-to-video animated sequels)
2 Fast 2 Furious: See the previous, same deal.
Charlie's Angels Full Throttle: Bad

Come to think of it, we may have already seen the best of what this summer has to offer as far as sequels. And then there is this, which makes my head hurt just thinking about it. How does an idea like this fly? What were they smoking when they decided to waste any amount of money producing this?