The non-standard features like Steam multiplayer and mods Workshop that lock out non-Steam users pretty much forever are IMHO even worse than the 1-year exclusivity (as long as it's not retroactive).
Steam multiplayer? You mean the friends list? Steam doesn't provide hosted mp servers for games. Mods are made by community members that can be released anywhere. Workshop is just a distribution channel.
Battle.net and origin are lock in services. Nothing about them is open to the public or accessible. Steam is pretty open and reasonably successful at it.
Mods can be released anywhere, but in practice, usually aren't !
Steam might not provide hosted MP servers for non-Valve games (are you sure it doesn't ?), but we've even seen older games having Internet multiplayer removed to the exclusivity of Steam's own "VPN" : Dawn of War 1, Civilization 4 (for which they also managed to break most mods for no seemingly valid reason)...
In my experience they aren't different at all. Steam Workshop is just a distribution platform for mods. It's a mod installer and manager. It doesn't stop you doing it manually.
Steam doesn't host multiplayer servers, but it does offer services for match making, lobbies, voice chat and P2P networking (including the use of Valve's network as a relay).
Also, Battle.net is in a somewhat different category, as, as far as I know, is only used for Activision-BLizzard games ? (No possible monopsony issues when it's the same company !)
