The Paper Mario series has long followed an odd trajectory, filling in the holes left in the wake of new trends on Nintendo consoles. The original N64 title and its GameCube sequel worked to satiate both fans of 2D Mario games and fans of turn-based RPGs, who both felt a bit left behind on those two consoles. If you hated polygon-based Mario and his adventures, and/or felt lost and alone after Square stopped publishing the numbered Final Fantasy games on Nintendo consoles, you could always turn to Paper Mario to try and get those needs met.
On the Wii, Paper Mario went even further towards the original Mario titles, which as history would show, had a ravenous audience that was starving for more. Super Paper Mario ditched the turn-based combat for a gameplay formula that felt by and large like a traditional 2D platformer, albeit with four playable characters, an experience point system, and the ability to explore the X, Y, and Z axis for a limited time (and all with minimal use of motion controls). Once again, the Paper Mario series was willing to move in the opposite direction that the majority of Nintendo was headed, while still doing its fair share of new things along the way.
Now that the New Super Mario Bros. series have taken back the reigns of 2D Mario proper, Paper Mario: Sticker Star has been left without a default direction. Thankfully, that's allowed the series to return back to a style of that Nintendo has been less and less fond of in recent years. The game ditches many of the RPG tropes associated with the series, while bravely asking the player to work for every ounce of success they achieve. Sticker Star refuses to hold your hand. That might make some Paper Mario fans feel uncomfortable, but will surely please naysayers who have called out Nintendo for going soft in their middle age.