Dessimu: Asking only out curiosity. Note: I am a casual player, and pretty much an insignificant every day consumer. How is DRM ruining your game experience if you buy the game, download the game both from GOG and Steam alike, install it, accept to whatever terms they give you, and in the end you use the product as it was intended to use - you simply play the game and uninstall it afterwards?
I hope it's clear that DRM can still be evil as a concept or as a potential risk, even if nothing bad happens right now. Evilness is not limited to practical considerations, although the practical impact is certainly important.
Basically you ask what exactly I don't like about DRM.
When I buy a game on GOG, I download the installer, I backup the installer (I actually really do, it's not a big thing) and then, whenever I feel like it, I install it and I play it and I uninstall it and I install it again. It works quite well and give me a rather strong sense of possession (although I know that possessing digital goods is a difficult concept). I can install it at any time on any of my computers and I know that if it worked in the past, it will work again.
With Steam I cannot do that. I have to ask every time I want to play or install for permission from Steam to do so. Did it happen in the past, that Steam gave no permission, you may ask. And indeed it happened only very rarely for me (and could be rectified after some time in the cases where it didn't work right away). However, I don't like them checking up on me. I kind of fear that Steam might cut me off at any time (they didn't). It feels less like a possession, more like playing at someone else's mercy. There is a certain risk, DRM will keep me from playing, although it's relatively small with Steam.
It's mostly the feeling of possession and who is in control that is quite different between GOG and Steam. With GOG I have the feeling that as soon as I bought the game, I'm done with GOG. With Steam I have rather the feeling that I pay for the possibility to play the game but I still need to rely on them.
Does it make a big difference for the casual player? I don't know, but I know that it makes a difference for me. I don't want to be part of DRM in computer games and in a way it's kind of ruining my experience.