Falco, as his name might imply, is a falcon. Also, Fox is EXTREMELY powerful when used correctly, and Falco has a "spike" (as in, a Down A move that can blast someone into the pit with ease) much akin to Ness's in the original SSB so he's also really good if used correctly.

Luigi is different enough from Mario to be usable, I think. I like him being in there.

Shadow is the black hedgehog, he's basically "evil" Sonic. And he's SO much cooler than Sonic. I had thought that Nintendo had the rights to them, except that Sonic Heroes appeared on all consoles, so it may just be shared.