I would recommend youtube. I played the first, and if I recall correctly, watching a video of the game could adequately and accurately sum up how the game plays. Attacks originally were done with different waggle motions and buttons, but it never grew too complicated.
If the sequel is anything close to the original in control, then you should definitely look for a youtube video, and you'd be able to see how things are from there.
As far as the first went, it's major redeeming quality was multi-player. Aside from that, it's a dungeon crawler with repeat enemies, but unique bosses. You walk from area to area on the map, clear the rooms of enemies, and finally advance to a midboss or a boss.
I'm not sure how unique characters are this time, but previously, it seemed like attacks between characters had several similarities shared among them.
Anyways, good luck! And if you get it, let us know what you think!
Also, GameFAQs.com has a message board for every game, and while you might not be familiar with the common ideas and characteristics of the posters, they still might reveal some about what's different between versions.