I've only just found out about this awesome game, and it makes me mad I'm saving money for a trip in France and another one in China next year or else this in addition to other existing games might just have made me buy a Wii.
Anyway, from the previews I've seen, this game kinda follows the previous Paper Mario games but this one is more of a platformer rather than an RPG (though it still keeps things like items and XP). The coolest thing is Mario's ability to switch from 2D to 3D and thus see the world from a completely different angle, showing things that were previously hidden. It's though to explain, and you really need to see videos to understand.
Case in point:
http://www.youtube.com/results?search_query=super+paper+mario&search=SearchFor example, in one video, Mario's running up a hill on which giant round Twomps are rolling down. It might seem hard if not impossible to avoid them by jumping but *surprise*! Switch into 3D and you realize that the Twomps are really thin while the slope is quite large, and you can just walk around them!
(Incidently, it reminded me of a cool super-power I imagined once after finding out about Tesseracts and reading an article about a hypothetical four-dimensional world. Like Paper Mario, the guy would be able to switch into a higher-dimensional existence and thus be able to see things and move/act in ways impossible before, although how exactly our world would look from such a point of view is anyone's guess).
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"With the first link, the chain is forged. The first speech censored, the first thought forbidden, the first freedom denied, chains us all irrevocably." [...] The first time any man's freedom is trodden on, we're all damaged. - Captain Jean-Luc Picard
B*tch, meet reality. Reality, meet b*tch. - Me