I haven't played either Super Paper Mario nor Thousand-Year Door in a while, so my recollections are pretty vague (especially with SPM). Please, bear with me.
As many have mentioned above, the likehood of SPM coming off as disappointing is higher if you expect a real follow up to TYD, but that doesn't mean it's bad, it's just good in a different way. The battle system is one of those things that might end up being a turn-off if you expect one that requires as much timing and reflexes as TYD's system did; in all truth, I don't remember using many items other than just the ocassional mushrooms and syrups along the way, and that was mainly because of special skill abuse or clumsy slip ups. The mix between real-time battle and the fact that enemy health points in Paper Mario have never reached absurdly high numbers (unlike many other RPGs I've played) lend themselves to make even the boss fights pretty easy. You must get it by now.
If you want some advice: I think it's better to see it as a Mario platformer with the scale, interaction and exploration of an RPG rather than part of the Paper Mario series. It really does a great job at that, in my opinion.
The sense of humor is really spot on through most of it, too. I guess it's the sort of game where humor makes up at least a third or fourth part of the reasons to finish it. But I haven't really seen it mentioned that the story wasn't really bad either. It's got some pretty intersting characters like Count Bleck's people (if you're into colorful and excentric casts, that is).