The idea that hardcore gamers just buy the same stuff is laughable coming from any Nintendo fan. Nintendo releases the same stuff again and again and everyone eats it up. NSMB Wii is the biggest Wii game to come out in a while. Almost everyone on this forum loves it, including me. But it is such a play-it-safe conservative cookie cutter title. Next on the horizon is Super Mario Galaxy 2 which sounds like a glorified expansion pack. Nintendo also recycles content like the NPC games. Wii Fit is all innovative but its follow-up is Wii Fit Plus which is such a minor update.
You can crap on GTA and Halo and God of War and all this but it's no fucking different at all. The only reason those games are crapped on is because they're NOT ON THE WII. Aside from the Wii___ series Nintendo has stuck almost entirely to really old franchises. Meanwhile Sony, who gets crapped on, has Uncharted, Resistance and Infamous which are all brand new IP for this generation. And even stuff like God of War, Killzone or Ratchet & Clank debuted only last gen.
Do core gamers just buy the same stuff? Well on the Wii the same stuff is all you can buy! Yeah I've got Mario and Zelda and Metroid but that's what Nintendo provides. That's ALL they provide that isn't aimed at non-gamers. The new IPs aimed at core gamers are actually on the other consoles. Sequels are going to sell and that's no different then the film industry. But that doesn't mean when new stuff is available it is always ignored or is destined to be ignored. More sequels are made because there's less risk and less effort involved.
The reason I don't own any of the Wii series besides Wii Sports isn't because I don't like original games. It's because I've tried these games and they don't interest me because they're too dumbed down. They rely on a novelty that wears off for me in a few hours. They lack substance. They're not worth the money to me. And that's all it is. It isn't even because it's aimed at a different audience. It's because the design used to appeal to that different audience results in games that I find BORING.
And even though there is no right and wrong in regards to liking games, if certain games are popular then that's what types of games will be made. The gaming industry is very much about trends. Something catches on and everyone copies it. So if I don't like the trend, genres I like get ignored and genres I don't like are made in abundance. If the novelty-over-substance of Wii Sports didn't catch on the Wii wouldn't be full of casual-focused trash. Maybe you like that stuff, but understand that if someone doesn't this really sucks. If Nintendo was making games you didn't like on a regular basis would you not be somewhat hostile about that as well?
As for casual gamers being more picky, hey, everyone is susceptible to some form of marketing. Cores, casuals, everybody. Sometimes it can be as simple as making a game that the person knows they'll like.