The Rift isn't a walled garden. Most Steam games are compatible, you can freely develop on it, run free software, use WebVR, etc... You also don't need a Facebook account.
The store itself is not open but you don't need to use it. In fact, with everything else being equal, it is recommended to buy your games on Steam, this way, if you get another compatible headset (like the Vive), you can still play your game.
As for playing our current games in VR, it is not that simple. VR require a specific control scheme in order to be enjoyable. For example, camera movement that doesn't match player movement can make people sick. For that reason, teleportation-based movement is often preferred, and gameplay has to be adjusted for it.