I can't really say what makes a good sequel, as there aren't too many. I know what makes a bad sequel though....
1) The same jokes over and over again (Ace Ventura II)
2) Making the movie just to make some more money (Jurassic Park II, III)
3) Thinks the audience hadn't seen the first part and rehashes the entire plot, wasting much of the movie (I Still Know What You Did Last Summer)
4) Tries to be as innovative and/or original as the first one, failing on both counts (Matrix II)
5) Rests on the laurels of the first one (Disney Rehashes... any great Disney Movie with a II behind it (Except Toy Story))
6) Same plot, different location (Die Hards)
7) Weak plot (Temple of Doom, Star Wars Prequels
8) Same characters, different cast (Dumb and Dumberer, Aladin II)
What makes a good sequel?
1) Good plot that builds off the first one but is independent on it's own. (Godfather II, Toy Story II)
2) Continues a great story (Empire Strikes Back, Return of the Jedi, LOTR -- TT)
3) Characters grow through both films (X2)
4) Takes whats good in the original and adds great characters while taking small tidbits from the first one and tying up loose ends (Indiana Jones and the Last Crusade)
My two cents.
Tony