I agree with a lot of the previously mentioned games. Another one I wanna throw out is Just Cause. It didn't have "one or two" flaws, it had a LOT of flaws, but it just had one mechanic that, had it been placed in a more intuitive freeroaming world, would have sold the game to me at about $100 a playsession.
Basically, you get a retractable parachute and grappling hook. The parachute is more of a glider than a parachute, basically take it out when you have a lot of velocity in any direction, and you can use it to float horizontally over a whole forest. Then there's the grappling hook, which is pretty much useless on any solid objects but can easily connect you to any vehicle, and works simultaneously with the parachute. This results in hanggliding sessions with cars on the ground, and the ability to steal a helicopter just by jumping in and nailing the guy. Oh, and you can jump from a plane to another plane with a simple button press.
That game was then let down since the world it was based in was a bit retarded; all the world's police hate you no matter what, and there's waaaay too much forest, pretty much disabling car chases anywhere but the highway. Just with the featureset it had, I could think of about a hundred "OH! OH! Andyouknowwhatelsewouldbecool?" ideas.