The combat system has been changed in such a way that fighting battles are now largely pointless: most of the time the sole reward is coins, which in this game are really only useful for battles.
There are fewer humorous moments in my opinion, and no Partners to speak of (Mario does everything). The only character that accompanies you is pretty dull, and fairly annoying: think of a bossier Navi.
The Thing system is also more frustrating than fun, as you have to know ahead of time which Thing Sticker to keep in your (limited) inventory. This is especially true for the boss battles, which become exercises in monotony (if not futility) if you didn't bring the right Thing Sticker, AND decide to use it at the right time. The first time I fight a boss, I basically consider it lost, as I'm there to learn the pattern instead of actually win. For added headaches, Thing Stickers are almost always larger than normal stickers (and thus take up more inventory space), and they're either expensive or require tedious backtracking to acquire.
The pacing of the game is uneven. World 3 in particular just drags on for an eternity. In fairness, the series has a history of sharing this weakness, so it's wrong to single out Sticker Star.
On the pro side, it's the shortest Paper Mario game, and is pretty well suited for portability.







