In order of importance:
Price: Main reason for the Wii's success this gen. Even though it undeniably has the weakest library of games, launching at $250 while the PS3 was sitting at $600 and the 360 was $400 or so (not sure exactly) gave it a massive advantage. First place consoles have always launched at $300 or less.
Games: I think price is more important (especially in the first year or so) but games matter a lot down the road. Once consoles are all at the mass market price like $300 or less I think is when games become important since price is less of a factor anymore.
Advertising/Endorsement: Another reason for the Wii's success is every single talkshow including Oprah endorsed the Wii and it had good advertising. In comparison the PS3 got ripped on by the media for being overpriced and a delaystation, so the Wii really picked up steam when Oprah personally endorses it.
Innovation: Important, but clearly not mandatory for success. SNES didn't really innovate, it was just double the bits of the NES, PS1's only innovation was the switch from cartridges to CD's, PS2 again only innovated by switching to DVD from CD...the Wii is basically the first winning console that relied on innovation.
