Matewis: Mount and Blade : Warband. That game had a firm grip on me with its medieval claws for longer than I'd care to admit :P I had just started to build my own kingdom, when I realized how long it would take to take over the entire map, at which point I quickly uninstalled the game. Wicked fun though!
Crewdroog: that is a fun game, but the learning curve for fighting is sooo steep. So much so, that when I put the game down for a bit and went back to play it, it was like I never had before. The thought of relearning all that was just too much and I haven't picked up the game again.
Yeah, some serious skill goes into the combat of that game, and even more so if you enable directional blocking. Still, even with it disabled, you need to be extremely precise with your attacks from horseback.
Once the sequel releases, and as soon as I get a proper gaming rig again, I'm jumping straight back into the action :D
I just hope they revamp the whole siege system.
Oh and I remembered another game I had to uninstall, Jagged Alliance 2. Still want to finish it one day though.