Throughout the game, characters get slightly upgraded attacks, in addition to the usual short-term powerups. All manner of unlockables are scattered throughout the levels, as well. For the most part, these take the form of new characters to use in the multiplayer arena portion of the game, which is separate from the campaign. We didn't find this mode particularly fun, especially when compared to, say, Super Smash Brothers Melee, but it does add a bit of value and longevity to what is otherwise a fairly short, straightforward experience.
Perhaps the best part of Teen Titans is the story. It's hardly an Alan Wake style narrative, but for fans of the show, there are lots of jokes and references to previous events. One of the best things about the cartoon was that the characters often break the fourth wall, talking directly to the audience. This happens in the game as well. Some of the levels are outright bizarre, most notably a level controlled by a villainous magician, who summons giant evil bunnies as minions. (Yes, it's as fun as it sounds.) My kids really enjoyed the game, and played through it with me once, and again on their own several times.
As a previous-gen title, Teen Titans can probably be found very cheaply. Wii owners will have no problems playing the Gamecube version, nor will PS3 owners have trouble with the previous-gen version for Sony's console, but Xbox 360 owners are out of luck. The Teen Titans cartoon is regularly repeated, so many kids today will likely be familiar with it. For young fans of the show, the Teen Titans game is certainly worth a look, though older gamers will likely find it merely average.