A lot of the games you guys just listed didn't exactly get stellar reviews.
A game like No More Heroes, for example, did some really neat things, but it's not exactly a "hidden gem." The title got decent advertising, it just wasn't that great of a game.
Okami, Sam and Max, geometry wars, all exist on other consoles and are reportedly better or cheaper on the other consoles (though, of course, differing opinions on "better" are valid).
Fire Emblem, Umbrella Chronicles, and the Trauma Centers (which is a great game, btw) have all appeared on other consoles, perhaps in a slightly varied form.
Games like Sonic and Super Monkey Ball may be worth renting or checking out, but they're certainly not hidden gems either. I dunno, it just seems like good games on the Wii always need an asterisk or a footnote saying "certain parts of this game are incredible but the rest is garbage" or "done before/better/cheaper on another system."