Well, since you mentioned DRM and return policy, let's not forget that GoG has no DRM, and unlike GoG's 30 day return policy Steam didn't even have one until Australia threatened to ban them.

And the Workshop and Steam MP might be very convenient for some, but are walled garden features that ought to be boycotted.

P.S.: More of a monopsony than a monopoly :

https://medium.com/@KingFrostFive/steam-monopoly-monopsony-4...