Something what ruins games for me is repetitiveness. It makes games boring and drains any will out of me to play the game.
Galactic Civilizations II had this problem in big-ass skirmish galaxies; Colonize, develop, expand. And once in a while I'd fight with an enemy. But once you've got your empire functioning properly, there's not much fun in it anymore.
I recently bought "Sins of a Solar Empire: Trinity" because the price was low, but after a few days I stopped playing. All I was doing was develop my planets, research tech, attack and expand. There was no variation in it.
I'm not asking the impossible. What I'm asking for is stuff to keep me busy even if I've been playing the same galaxy for days/weeks. Give me side-missions, lots of them and varied ofcourse. Let the goverments of the worlds you own give out side-missions/tasks to hunt down people or some such. Stuff to keep me busy! That's what I need to enjoy a game.
Okay, I'll admit, there are some games that are repetitive but don't get boring (for me that is) but turn-based games have never done that for me.
Proper single-player campaigns are what make me happy because you have story-driven missions in those. GCII was great in that.
I should note that I usually play at an easy level. The reason is that there are enough annoyances in games for me without getting killed every x minutes or turns. So how I experience games might differ from those who play at harder levels.