Until recently it was not possible to import or export 3D models, so that excluded cosmetic mods.
The game continued being updated with features and content after release, which was good for the game, but not necessarily for mods. Presumably after the next update (news about that next week) it should be feature complete (so no more breaks in compatibility) and more resources can be put into the editor (which has continued to be updated).
You can not easily have multiple mods enabled, or update/change mods after starting a game.
That said, there are at least a couple stand alone mods nearing completion.
