-We could see an FPS rise in eSports prominence again
-Mods built on top of one game create tons of variety accessible to one player base
-Mod developers have an easy way to become recognized game developers
Also, not related to the old Quake / UT / Half-Life scenes, but the marketplace for assets will be on top of a game that isn't Team Fortress 2, so there's much more creative latitude once mods are released (disclaimer: I love TF2).