1. For the most part[*], publishers on Steam use DRM, from kernel-level DRM which can crash and pwn your computer, to Valve's relatively lukewarm Steamworks., whereas GOG primarily (exclusively?) sells DRM-free games.
There are lots of ideological and practical concerns with DRM, I won't list them here other than to say game players want to be in control of their machines and their experience, not let game publishers control their machines.
2. Steam policy is that you can only run the very latest release of a game (it will update when you go online, and you can't remain offline forever). It takes away your choice to reject publishers bad updates - for example, when 2K forcibly added their marketplace/launcher malware to Bioshock games, breaking them on Linux, Steam was their henchman/goon forcing it on everyone.
It depends on the game and what the mod maker decided.
I made the mod for the Divinity: Original Sin that changes a few bytes in the game XML files to allow for 4 players in game instead of just 2, since it was mostly supported but probably removed for console porting simplicity. This is a braindead simple mod that just needs to find some XML tag inside the embedded EXE/DLL file and update it. I didn't even have to update any checksums, etc.
When I published the mod I chose to target the hashes/offsets of the Steam EXE since that was what everyone had. So, while I didn't target the workshop (as this modification could not be done with it) I did target Steam end-users.
Gabe's done a decent job keeping the capitalism at bay, but he won't be around forever. Even if you support steam and what valve is doing now, that may change in the future. Diversity is good for consumers.
