Flat.
I'll confess that I didn't come into Paper Mario: Sticker Star with a very open mind. It's the last 3DS game in my backlog, and I really wanted to pack the system up before the end of the year.
That being said, the 30 minutes I spent with Sticker Star were enough for me. While I might not call it an outright "bad" game, it certainly didn't offer anything I'd call "good," either.
An interesting story? Nope. This narrative setup is less complicated than many of Mario's platform games. That is to say, in Sticker Star, an opening cinematic establishes - almost wordlessly - that Bowser interrupted a Mushroom Kingdom event, a magic macguffin split into six pieces, and Peach got herself kidnapped. So ... I guess it's time for Mario to do that thing, you know, the thing he always does. With the princess saving.
His only traveling companion is an annoying talking crown, who provides some gameplay tips a'la Navi the fairy. And since Mario himself remains mute (as is tradition), there isn't much in the way of dialog. NPCs have some light-hearted throwaway lines, but the humor feels underwhelmingly mild.
As far as I remember, Paper Mario: The Thousand-Year Door had a surprisingly complicated plot. And while the story of Mario & Luigi: Bowser's Inside Story wasn't very good, I respect that it at least existed. Sticker Star, in spite of its predecessors, clearly never intended to tell a story at all.
So does it make up for that with some thrilling combat mechanics? Eh... no. Its sticker-based combat is based on, well, your inventory of stickers. Mario doesn't have any default attacks, or abilities that he can learn by leveling up -- because there is no leveling up. Nor are there equipment slots, stat upgrades, or ... even character stats, really.
There are just the stickers, which you find in the world, or buy with coins; and which are consumed when you use them.
Your mileage may vary, but I found this combat system to be particularly shitty. That the stickers are consumable makes me worried about using them incorrectly, or running out of "good" ones; and the lack of permanent character upgrades makes me concerned that battles won't change very much as the game goes on.
Sticker Star does, at least, retain the Mario RPG mechanic of real-time button presses bolstering your offensive and defensive moves. (Even though the annoying talking crown didn't explain this. I guess it's a good thing I've played the previous games.)
But it's otherwise thrown out a significant portion - the storytelling, the combat depth, the character growth - of what made the Paper Mario games, and the Mario & Luigi games, compelling. What it replaces those elements with feels hollow and insubstantial.
The message I got from Sticker Star, loud and clear, is that it doesn't want to be an RPG. It would rather be an adventure game in the Mario universe, that happens to share a paper-cutout aesthetic with some other games that were RPGs. And that doesn't sound interesting to me.
As a tangent: what made me shut the game off, and formally retire my 3DS, was when I filled my sticker book by accident. I didn't even know it had a size limit - let alone one I could reach so quickly - and the sticker I tried to pick up just ... disappeared. (A precious, expendable sticker, gone forever, because I didn't realize that I'd be unable to carry it. Tragic.)
Progress: Just left Decalberg.