The biggest thing the Wii has on either the PS3/360 and its games is its infinitely larger potential to have a worldwide or cultural phenomenon.
These games like Assasin's Creed, The Darkness, etc. might be really good games, but what would make the world go crazy about it? People went crazy for Halo, and people went crazy for GTA much like they did for Super Mario 64.
Since then every huge hit that has been released as been for a Nintendo system. Nintendogs was unproven at the time, but everybody could see how potentially big the games could be. That potential was high and it was fulfilled. The same deal occured with Tetris years ago and happens to Brain Age now. It's what's happening with Wii Sports, and it can easily happen with Wii Health Pack.
Nintendogs appealed to a huge audience. It appealed to females, dog lovers, people that actually wanted a damn game for their DS at the time
lol ), and those that love the idea of sims in general.
Brain Age appealed to a huge audience as well. It appealed to older people, it appealed to females, it appealed to parents/kids because of the thought that it was beneficial for one.
New Super Mario Bros. also appealed to a huge audience. It was very pick-up-and-playble. It was a perfect fit for a handheld the way the game was designed. It appealed to nostalgic Nintendo fans and it appealed to the younger audience that enjoys colorful and easier games.
All three of these games have sold at least seven million worldwide. When I look at stuff like Heavenly Sword, I think that the best case scenario for it were if it were likened to a Goddess of War. But even God of War wasn't HUGE.
The potentially huge sellers I see are GTA IV, Halo 3, and Forza Motorsport 2, for pretty obvious reasons. But GTA IV won't be like GTA III just like Super Mario Sunshine didn't get results like Super Mario 64 did.
It's easy to see how games like DDR, Guitar Hero, Brain Age, Wii Sports, Pokemon, etc. became huge for gaming. I don't see much on the horizon outside the ones I listed for 360 and especially PS3.