I've never played KOTOR, don't really like Star Wars. I do like Star Trek & other Sci-Fi though, & RPG games.
I wasn't really that aware of Mass Effect before I got it, but I bought a 360 October last year, then Game were doing a promotion around Christmas time & both my Mum & I were running out ideas what to get me as a present (since I bought myself every 360 game I was interested in when I bought it), so I thought I'd try it out.
I was also attracted to the character customisation aspect of it.
Also I'm glad I remained pretty much naive about Mass Effect until I played it, with low expectations it was hard to be disappointed & I thoroughly enjoyed it. Thinking about it, it's the one of the only Sci-Fi RPG's I've played, except Fallout:Brotherhood of Steel, but that doesn't really count.