GeraltOfRivia_PL: Also i don't know why are you so highly rated on this particular comment since what you said is simply untrue. GOG also has the upper hand over Steam in that they actually give the slightest fuck about optimising their games and getting them to run, even if they are third party. I haven't played Fallout 3, for instance, but from the reviews on Steam it seems that it doesn't work on any OS younger than Win7. GOG version doesn't seem to receive that many complaints. So this is certainly something good, because even if the original publisher/developers doesn't give a fuck anymore, you can probably still play your older games without having to install one hundred million mods to run it. Again, it's not something that common on Steam, but it does happen.
I have Fallout 3 on Steam and it works fine for me on Win 8.1. I might have had to do some minor tweak, like copying a DLL file into a folder or something in that vein, but it wasn't a big deal. In general, you are right, GOG puts extra work in testing and trying to fix games, while Steam does not, at all. In practice though, I think I have yet to see a case where GOG came out as a real life saver in this regard. Obviously, they can't do magic. They mostly just seem to apply 3rd party patches and fixes that you can find on the web yourself, with minmal research, and while this means a little less work for the average user, there is no guarantee that the game will run better than the Steam version on any individual PC and that you won't have to look for additional solutions anyway, because it does not run that well for you right out of the box. I had high hopes, for example, when Saints Row 2 was released here, because I thought GOG would fix the performance issues and the frequent crashing and all that, but in the end, I didn't notice much of a difference to the Steam version. Even after applying all the fixes, the game is still quite broken on PC, be it Steam or GOG version.
So in theory I agree that this is something laudable in GOG that Steam does not offer at all, but from my experience, it sounds better than it actually is. At least GOG makes it a little clearer on the shop pages though, when a game is not supported by a certain OS.