Mafia 2 was more amazing than i expected. The story line is great, and even though it's a linear game set in an open world, it still works well. I would suggest it if you're looking at it.

The mass effect series, I could not recommend more. I'm ver picky about RPG's, but mass effect is an absolute good. Just cause 2 is a good LONG game, if you do all the story and side quests, it's also a hell of alot of fun. PAYDAY: The Heist is a highly underrated online co-op fps. Extremely fun.

As well, Team Fortess 2 is free, go download it if you don't have it. I've put 446 hours into it and still love it.

Hopefully I helped, i went through my steam list and posted a few games you may have liked. Sometimes it helps to look at what is popular on steam, and dl demos if you're unsure.