I'm basically the opposite of Mark CMG in that I think the best games are ones where the mechanics were designed directly in support of the setting they were meant for. I find most 'generic' mechanic systems to not be as good as ones that were designed specifically for the settings they represented.
Sure, I could use d20 Future or GURPS Space to create a game for the Star Trek or Star Wars settings... but I find playing both of those specific rpgs to be much, much better for the task at hand. And this is why 7th Sea is hands-down a better roleplaying game than Swashbuckling Adventures (AEG's d20 ruleset for their Theah setting).
Sure, I could use d20 Future or GURPS Space to create a game for the Star Trek or Star Wars settings... but I find playing both of those specific rpgs to be much, much better for the task at hand. And this is why 7th Sea is hands-down a better roleplaying game than Swashbuckling Adventures (AEG's d20 ruleset for their Theah setting).