I play games because I like to do everything, I like to try everything once. Games give me a wider range of things I can do and not have to actually go find time to do them. That and read books both are good at that.
I play games because I like to see how they evolve, I like to see everything someone is doing to out do another. The originality of games is what keeps me coming back. Even if someone does something old really really well I love it.
But games that set themselves apart by doing something original AND doing it well are what really get me.
A few examples
Natural Selection: (imo by far the best humans vs aliens WITH an FPS/RTS/command element two rarely done concepts, an even rarer combination and it blended fantastically)
Red Faction: and the newest iteration Guerrilla (The geomodding in the original was ahead of it's time and made me sad when no one else saw it as a bonus, extremely original and a blast to use. Sadly missing the Guerrilla, however the ability to disassemble buildings is a blast)
Shattered Horizon: pretty standard fare shooting except for one thing IT'S IN SPACE, while the controls weren't the best and the game takes a bit to get used to the game itself implements the 3rd dimensions fantastically and even includes a sound simulation and a running silent mod to sneak up on your enemies. Absolutely brilliant.
These games aren't the best at what they do perhaps, but they have something many other games don't originality that makes them fresh and new something that changes the game so it isn't the same each time you play and let's you invent adjust and reimagine ways to win. This is fun to me being clever and out smarting your enemies is what I love. to me Halo 3 and gears of war are bland shooters where repetition leads to victory. Give me something that makes me think.
I love games because each one is different and I want to try them all. OM NOM NOM NOM!