Switching to steam only has allowed us to make the game way better. This is mostly just because we've had a ton more time to work on the game itself.
Adding GOG right now would be really difficult. It's actually going to be getting very slightly easier in February but we still don't have plans to do it.
DRM is usually first in the discussion and we don't use steam for its DRM, it's all the development, distribution, patching, testing and marketing tools that we need from it.
One of the main issues we see people having with steam is that they don't want to need to be online to play the game. If this is the issue for you (and it might not be) then, good news, with 1.0 we are adding a true offline mode. If the game is launched in offline mode, you won't have access to any of the online features and it won't call home at all so you can play it on an airplane or remote island if you want. You can also still go into online mode later if you choose.