The chief problem with Steam as it currently is setup is that you have to go online in order to initiate off-line mode. Basically, if you are not already in off-line mode, and your Internet connection goes down, you can't play any of your Steam games.
Exit Steam, disable your network adapter, restart steam, see what happens, learn something new ...
All you need to enter offline mode is a steam client which gracefully exited the last time you logged in while online (you can remove the internet connection while Steam is still running as well, won't prevent you from entering offline mode). Only when credentials get screwed up because the client is force closed or your computer crashes will you have trouble entering offline mode (ignoring bugs).
I think there are good reasons to still go with GoG (my preference goes direct from developer > GoG > Steam) but not this FUD.
