I was reading a story about the hostile takeover bid from EA for Take 2. In a gross oversimplification EA was depicted as the company making nothing but sequels with only minimal improvements over last year's version of the same game, while Take 2 was described as the small innovative company launching lots of original games. Now apart from the obvious holes in that version (EA's next big game is the original Spore, while Take 2's next big game is a sequel, GTA4), I can't help but notice the fact that EA's less original sequels are selling a whole lot better than the original games of Take 2. It is as if the customers don't want original games. And I think that has a lot to do with what people are comfortable with.
Video games, and especially MMORPGs, have become rather complicated. Apart from the "casual" games like Peggle, which anyone can understand in under a minute, most video games now need quite some time to "get into". Just look at whatever MMORPG you are playing right now and count how many different on-screen hotkeys, keyboard shortcuts, and other control elements you are using. And for games like WoW that is just the bare minimum, most players enlarge the number of controls with addons and macros. This creates a barrier to entry: Very few people play several MMORPGs in parallel. Not only because each MMORPG takes too much time, but also because in game A you press "I" to open your inventory, and in game B you press "B" to open your bags, and if you switch back and forth a lot you'll quickly get confused and mix up the controls. Other types of games try to counter that problem by working with de facto industry standards, most first person shooters use very similar controls. And since Diablo it is hard to find a game in which the health potions aren't red and the mana potions aren't blue, with health / mana bars in corresponding colors.
But because it remains difficult to learn a new game from scratch, some people prefer their new games to be nearly identical to their old games. And that is why the umpteenth version of Madden is still selling so very well, and why there are sequels everywhere. It is also why World of Warcraft is hugely successful, while not being terribly innovative. That of course poses a problem for games like Age of Conan, who try to introduce a completely new system of controls and combat. MMORPG combat hasn't changed much since Everquest, and I'd really love a new system to succeed, but there will always be a lot of people unwilling or unable to learn a new system of controls.