First off, Nintendo is one company which has produced some of the most games of any company in any gen out there. In this gen in particular, they are even getting flak for producing MORE sequels and prequels than last gen for games like Mario, Kirby, Pokemon and of course, their new 'Wii' series.
Second of all, the argument can be made that they take longer on 'certain' titles based on quality control and because they're trying to develop new things. Using your examples from the original post, MOST of those examples are sequels, but MANY of them are not following a similar design strategy as Nintendo titles. Are you really going to say something like Gears of War 2, Ratchet and Clank 2 or Blue Dragon 2 are going to be as unique and well...innovative of jumps in sequels as say Metroid Prime 3 to Metroid Other M or The Legend of Zelda: Twilight Princess to the next Zelda Wii. Or how about games we've already seen, like the jump we had from Mario Sunshine to Mario Galaxy or the previous Kirby games to Kirby's Canvas Curse.
The simple answer is, while not all Nintendo games follow this philosophy, the reason some Nintendo titles take longer is because they do completely unique things and are built from the ground up to be completely unique games. You can't completely re-invent Metroid or Zelda by simply taking the previous game engine and making a 'sequel' in under 2 years.
As an aside, not every company out there completes sequels quickly, as per the opening posters example. Someone mentioned Blizzard, which has been spending over 10 years creating a sequel to both Diablo II and Starcraft. And look at SquareEnix, which recently wrapped up development of both Final Fantasy XIII and Dragon Quest IX, whcih both took nearly FIVE YEARS each, and one was a DS title.
Six upcoming games you should look into:
