The way I understand it, D:OS was made with the idea in mind that users would be able to create their own adventures. "Mod" in this case wouldn't just mean "modification" but also adventure "module", just like with Neverwinter Nights. So it's not just something that's tacked on to the real game, only for those who don't like its looks or mechanics or whatever, but a concept of the game itself to encourage the story-telling to continue after you're done with the official story. If you want to stick to the original campaign and never try any custom adventure campaigns, that's your choice of course, but games like Neverwinter Nights are not as popular as they are because they have awesome official campaigns. IMO the people who prefer to stick to official content just for the sake of it being official miss out on a lot of creativity and originality.

Raze of Larian has confirmed that D:OS mods are just offline files that can be downloaded anywhere, say from a Nexus site like how good old mods used to work. I'm not sure if you can pull out mod files that are downloaded from steam workshop and use them on the GOG copy but I'm sure they will be mods around the internet especially Nexus.

that seals the deal for me, I'm going to buy it when I get more in the bank

D:OS modding is at an extremely early stage--as in, people on the Larian forum are still trying to figure out the basics with help from Larian's ongoing video tutorial series. Right now there just aren't many mods anywhere.

No dedicated hosting sites currently, although someone did offer.

I'd say the other way round: buy it on Steam, get it DRM-free on a future sale if you want to. The way I see it, unless you have problems with Steam (such as bad/no internet connection) DRM-free is for backup. If you want to play the game now, Steam is there and will be there for a while.

Workshop.. > GOG. Sorry for this one.

I know that this is coming pretty late but still I would like to thank you for all the feedback.

Yesterday I decided to get the game from GOG, inlcuding the DLC. I really hope that most of the content (okay, at least the good stuff) from Steam Workshop is going to be made available on other sites.

I'm not yet sure if multiplayer is going to work for me. I will test it in the coming days.