So, I think it depends on what you want out of the tag system. If you want it to be a global tag that tags a post similar to how they’re used on tumblr or something like that (I.E: Has meaning not specific to the community it is in), that should be separate from per-community tags, like they’re done on reddit. I think per-community tags should definitely be added, similar to how reddit does them (for a good example of how they are used, see /r/talesfromtechsupport). Global tags, I’m not as sure, and if they are added, I think they should be separate from the per-community ones. My hesitation for the global tags is that it will create meta-communities, similar to what happens on tumblr, which blurs the line between communities, which makes moderation a little weird.