The reason I am not a Steam fan (I don't hate it, just not a fan) is that I hate the idea that only ONE person can play any games in your library at one time, including yourself.
I am not talking about two people playing the same game, but rather one person playing one game and another (or you) playing a different game.
With GOG, there is no DRM so you never have to worry about that. Of course GOG does not get many just-released AAA titles .. except when the company that owns GOG develops them *laughs* .. but I have more than enough games in my library queued up for after I move that I can wait until they eventually make it to GOG.
Steam offers a lot of conveniences and if they did not have that one-person-playing-per-library-per-time policy, I would be okay with everything else.
For example, I have friends who play multiplayer games regularly but they constantly take breaks for various things. I want to be able to stay online in the game they are playing and play another game in my library while I wait for them to return. With Steam, you cannot do that.