I've been playing (and loving) the Paper Mario series all the way from its beginnings on the Nintendo 64, so I was excited to learn that this flattest of Marios was on his way to the 3DS this year. After playing through World 1 of Sticker Star, I'm going to lay out my first impressions of the game. Keep in mind, I may not have encountered some elements yet that could change the complexity of the gameplay.
First off, some back-story: The series began with a heavy RPG influence, including turn based battles, companions to choose from for your team, levelling up, and equipment upgrades. Paper Mario: The Thousand Year Door (Gamecube) continued that tradition and upped the ante with a great story that was tonnes of fun to play. The third Paper Mario (Super Paper Mario, on the Wii) almost felt like it was coming from a different place entirely, though. The equipment, levelling up, and companions were all there, but the turn-based battles were gone and the world had become almost entirely side-scrolling.
At a glance, Paper Mario: Sticker Star seems like a return to form for the series. The turn-based battles are back, and the world has regained its depth (though still mostly a side-scroller, similar to the first two games). However, Sticker Star seems to have been, for lack of a better word, highly simplified. Mario no longer collects XP when fighting enemies, only coins. The only form of levelling I have encountered are occasional HP upgrades in the form of hearts found either on mini bosses or hard to reach places in the world. The story seems simpler, with a very brief introduction and little explanation before Mario heads out into the world to collect the "Royal Stickers" (this game's Star Spirits/Crystal Stars/etc). Even the combat has been cut down by removing Mario's ability to select a monster to target (you are forced to attack the first one in line, unless you use an AOE attack).
Far from ruining the experience, however, I still find myself enjoying my adventure so far. This is probably because of the shiny new system entirely fresh to this iteration: stickers. Every one of Mario's attacks are dependant on stickers. Mario keeps a collection of all different kinds in an album, from Jumps to Fire Flowers to Super Mushrooms and every turn during battle, the player selects a sticker and performs whatever action that sticker dictates. I worried initially that there would be issues with running out of stickers, but the sheer volume of the things to be found around the world and towns means this is never an issue. Even on the comparatively long first boss battle I only used about two-thirds of my precious stickers.
Prognosis: if you're strictly in it for the RPG elements, you may be disappointed, but if you're a Paper Mario fan, you'll probably get a kick out of this game. I know I am.