The best kind of sequel: it builds on the first game and refines elements, namely the combat.
You topple foes through the combination of guns and plasmids (essentially, magic). The cache of guns is pretty standard issue. You have your shotgun, your rifle and a few other nice surprises. The plasmids are numerous and varied and most are effective. This gun/plasmid balance provides enough variety to support different approaches to the combat. The first game attempted this but did not succeed to this level because the trap plasmids were not terribly effective or fun. 2 gets it right. You like chicanery? There are plasmids and guns for setting nasty traps. You like the direct route? There are plasmids and guns to wage all out firefights. The beauty is that the game does not force either approach on the player. You can play every encounter different. Few games I've played include this much variety and manage to maintain balance.
The missions provide the framework in which you levy this destruction. They are not as inspired. Objectives get tiring toward the end of the game and always seem to boil down to fetch quests and level pull/button press quests. Kill these enemies, find this switch or hit this button and progress. The game does throw in some moral choice here and there to mix it up. But after 12-15 hours it starts to wear a little thin. And if you've played the first and invested that time already in Rapture, the feeling of "more of the same" is multiplied.
So while it does feel similar and is not quite the revolution the first was, Bioshock 2 is quality in its own right. It boasts a more refined combat system, greater variety of enemies and a strong antagonist whose ideals shape the world around the player.