Let me start by saying that I LOVE tags. Forget folders, forget categories, I am a tag kinda guy.
That said, you probably know what I like about WordPress: the built-in tag support for posts. Unfortunately there is no equivalent for pages. Or items in the media library. Yes, there are plugins ("Simple Tags" comes to mind or "Media Tags"). These hacks work reasonably well but the one thing that is missing, is a central UI to manage these tags. I want to re-name them, re-assign them, see what pages are tagged with that tag - the whole kaboodle.
I think WordPress needs a unified approach to tags, tags should be built-in and be universal to all types of posts, pages, links, downloads etc. I think there should even be a new menu item in the backend for this. This would really help WordPress to stay ahead of the competition while it is positioned more and more as a CMS. Tag clouds are one of the most prevalent types of plugins out there - and for good reason.
Think of all the amazing stuff we could built in our themes if tags were to be supported more universally. We could show links in the sidebar that go well with that post, pull images from the library that illustrate a page really well, embedd matching downloads for that product page etc.
I would be more than happy if someone were to tell me that all that tag goodness is in the pipelines and ready for WP 3.0. If it's not, please someone get to work on this. Let me know what you think!