To give some insights as to why such a seemingly simple feature takes so long. For it to work properly we need to make quite a few significant under-the-hood changes, for instance to support tags with spaces, but also to unify a number of different kind of things to make autocompletion not only work for tags but also for people etc. This has been more work than we had originally foreseen. Good news is that those refactors are pretty much done and we’re shifting to the UI work. It also prepares us for a number of other frequently requested features.
Having said all this, as a small two-man team we also have to take the impact of the yearly new OS releases from Apple in mind and spend time on preparing for those too. Which means the ETA has unfortunately slipped from before the WWDC update to the one after.