Trouble free purchasing / downloading installing.
DRM free.
ALL of my games (I only buy here).
Regular sales. With 200+ games it's not often I see something to buy that often, but rather than moan I appreciate the effort.
The helpful support team (in my experience. Only a few times required, always a satisfactory outcome).
The old games, the new ones,
Galaxy still optional.
The occasional freebies.
The game specific sub forums.

Mostly friendly and helpful community.
The regular and not so regular donaters to giveaways.
The upkeep of the giveaway threads and the "What did just update" thread.
Positive threads like these.

The DRM-Free games and the community (not sure whether to thank GOG or the users for this one honestly). As for the latter, I often find it hard to develop a sense of community on Steam due to the very large player base that resides there. Whereas on GOG, the tiny pool of audience has at least allowed users to encounter the same faces quite frequently and therefore can feel at home or at least develop a sense of belonging. One downside to this is that I find Steam game specific forums to have higher response rates compared to GOG's. But heck, that's fine by me.

This!

Alpha Centauri and Master of Orion 2 are what brought me here. The community and DRM - Free keeps me here :-)

I guess for allowing people like you to do their thing