I think the main thing I’m worried about is: we’re not really a tag-only setup: categories are important too, and we’re using tag groups with per-category rules. This doesn’t lend itself well to a big flat alphabetical list in the sidebar.
But I think I may have put this too negatively. 
on the one hand, there’s that worry, but the other side is that richer support and better tag UI could make things even better and easier, so I’m looking forward to it!