That said it will host content that workshop won't, so we all know where to look for that naked dregin mod
And there in lies where nexus value comes in. It maybe ,.. because of copy right!! that steam will not allow and Star trek mods where as I-Mod and this nexus may.
Don't think it is so much a copyright thing... more an "appropriateness" thing... like all those mods that turn RPGs into soft porn.
The steam workshop has lots of mods that are essentially taken from movies, tv shows (particularly anime), books, etc.
Again, I'm not partial to the workshop. I just prefer to maintain a mod in one place (because I'm lazy). But a modding forum could be really useful, maybe not so much for always updating/posting mods but for discussing mods, helping each other debug, etc.