Great gameplay is key, but Nintendo is really offering neither excellent gameplay, nor cutting edge graphics (within the confines of Consoles). They have a consoles that runs on pure gimmicks out now, using decade old hardware, that almost entirely lacks quality 3rd party support. Mario Kart and Mario Galaxy will sell regardless of how they look, that's why Nintendo milks the shit out of Mario. Their first party development is--and always has been--exceptional.
I don't understand why people always insist WoW looks like crap. If you crank WoW up it looks really clean, and pretty damn good. The only thing I can think of is because of the non-realistic feel the game is purposely going for. The game looks sleek, plays well on a variety of hardware, has very detailed environments, and excellent textures. What more can you really ask for? It might not look like TERA, but that's exactly where the gameplay makes up for it. As for Minecraft, it is and always will be a niche title, it's not something that can be marketed to the masses, and will always remain the kind of nerdy Indie Game PC Gamers love.
I don't think anyone is outright saying "GRAFX IZ ALL DAT MATTAS!?!", all people are saying is that they are sick of playing games that have decent gameplay and look like vomit. There are just not a lot of quality games on the Wii. It basically boils down to Zelda, Mario, Monster Hunter, and soon Xenoblade. The quality games on the Wii are primarily Nintendo-developed, and are so few and far between, most people have just given up on the system.