My wife wasn't much a gamer when we got together - she had a Playstation and a few games, but didn't play seriously. I think I got her hooked on Simpsons Hit and Run on my PS2, but her tastes were still fairly limited.
Actually that's pretty much where it stayed until she suggested we get a 360. I don't know exactly how it happened. We bought a 360 and this is when she made a commitment to gaming, with no real precedent to indicate why. There are certainly some games she hates (Mass Effect and Oblivion are far too slow paced) but for the most part her tastes are in line with mine.
I think the biggest change came when we were playing the Halo 3 beta via Crackdown.
She didn't like Halo - she'd watched me play it, but that was as far as her interest went. Maybe it was the online factor, that you could play against real people over Xbox Live... I don't know. She certainly hates the campaign, but she loves the multiplayer.
After that she played everything I brought home, and liked most of it - but she wouldn't play RPGs.
Then thatchanged. I still don't see her willingly loading up Final Fantasy 13 or White Knight Story whenever those come out, but she took well to Fable 2 and that became her gateway into Fallout 3 - which I thought would be a frustrating grind for her, but apparently not.
In short, my wife games more than I do, and in some cases is a lot better than I am. It's kind of frightening, considering how much longer I've been at it.