Good ideas, bad realization. I honestly would like to see another shot at it...
* Sticker System had potential, being a lite-Baten Kaitos type system, but lacks a real risk/reward element to it. I honestly was not that upset with Partners gone since it seems like a cool replacement, but badly implemented. I think if more of the Stickers summoned enemy characters, that would be a nice balance.
* It's the most "authentic" RPG to the mainline Mario canon, perhaps, and this could've been really neat if done right...imagine giving all the established characters like Birdo, Bowser Jr., Daisy, etc. the chance to actually have characters! Unfortunately, Miyamoto didn't let them do that so all the established characters that appear like Bowser Jr. (in his first RPG appearance, something I wanted for years!) come off as flat and more or less just random foes. Kamek perhaps is the most fleshed out, but barely. Paper Mario 64 DID THIS RIGHT by fleshing out the established cast like Peach, Bowser, and Luigi, and still feeling like it was part of the Mushroom Kingdom (which TTYD and Super both faltered on I feel), but also could do its own thing when need be.
* The World Map was pretty cool. I liked that, and with a good plot and more interesting locations, would actually like to see that in the next Paper Mario.
Honestly, I think the lack of an enticing plot is the biggest death-kiss to the game. TTYD and Super Paper Mario have a lot of questionable elements as well, some really tedious moments, but they both, imo, have great writing that made you overlook their flaws. Sticker Star doesn't have that, so there's no real push to see what the game has to offer once you realize a lot of it falls flat despite having a strong presentation graphically and musically, and some neat ideas.
Some people think they should totally abandon every single idea in Sticker Star, and those folks are wrong. What they should do, like any good developers should, are study what works, and what didn't work, and try and refine that. Imagine if every developer tossed out an idea after a weak first entry---we'd miss out on a lot of great, refined games.