Definitely, but there's been some useful food for thought here. I think the tag-limit suggestion is clearly the right approach to my question, and lets me see whether it's a real problem before I worry about it.

As I just replied to Dan, I very much agree about the UI for reusing existing tags. Not sure exactly what that's going to look like yet, but I may get fairly heavy-handed about it. Tagslop is such a common problem that I suspect it's worth very strong encouragement to avoid it.

And yes, I agree about Merge, although I think it needs to be viewed more holistically -- I suspect we're eventually going to want a Tag Management UI that lets you see the full tag hierarchy in your space and shove things around quickly and easily. Possibly even drag-and-drop, but I'll need to see some realistic examples before I start designing. Regardless, it's probably a year or two off.