Super Paper Mario for story, writing, and trying something new. Sticker Star made you use stickers to perform actions that could have been performed under normal circumstances (jump, hammer). Though thing stickers were cool, I was frustrated that Sticker Star had no real story, and had arbitrary limitations in battle (no attacking any enemy except the first one, no running away unless the game decides you can, no individual enemy health meter, etc.) I still like Sticker Star, but I now view Super Paper Mario in a more positive light now.
Edit: Another thing is that boss battles in Sticker Star are too scripted. The game intends for you to use a very specific thing sticker, after which the enemy's large HP will drop massively, and then you follow up with a simple attack to win. The only hard part about it is finding out which sticker to use at times, after which it isn't really even a battle anymore.
