timppu: [...]
I could be mistaken of course, but I think that one major difference between Steam and GOG at this point is that Steam has a hands-off and laissez-faire approach to the games. Valve does practically nothing to individual games, they just offer the infrastructure and tools to developers/publishers, and the devs/pubs are supposed to take care of almost everything from then on, making sure their games work with Steam and have Steam features and whatnot. This works because Steam is the dominant force (most users) and the publishers want to be there, no matter what.
[...]
It also works because most developers actually like this hands-off approach. One 'problem' with for example gOg is all the hoops you have to jump through, and a lack of control.
edit - to put it a bit wrong, gOg's philosophy is "confirm or die" whilst Steam's is "do as thou like". Pros and cons with both, off course.