I was more or less in the same predicament. I love valve titles, but I was ready to move on to other things. I started off with the Mass Effect series; the first one has a greater amount of rpg elements than the 2nd one, but the second game was overall better than the first. The story and possible outcomes of the ending for each game are amazing, and I would definitely suggest them. I then moved onto another bioware game called Dragon Age Origins; if you want rpg elements, it's certainly a game to shoot for.
Finally, I know it's not a single player game, but I just started playing a small-time game called Heroes of Newerth
. If you've ever played custom games in Warcraft III, you'd be familiar with DOTA; well, HON is an evolution of this game, which is essentially 5 heroes going on against 5 other heroes in an all-out battle. I may not be giving it justice, but it's a cheap, fun game if you're looking for a bit of a change of pace.