I have a lot of categories in this blog. 542, to be exact. I use them like tags, rather than a rigid hierarchy.
The WordPress Categories panel in the "Write Post" interface sucks with this many categories/tags. Having to scroll through a verrrrry long list to hunt for keywords is tedious. Here's a screenshot of Drupal's autocompleting text-entry interface, which makes it easy peasy to select existing categories/keywords and to add new ones all in one fell swoop. As you type keywords, an ajax call is sent to the blog, returning any categories that match the characters that have been typed, making it easy to select matches from a potentially infinite set of terms. This pattern is also used by del.icio.us very effectively.
Compare with WordPress' infinite list of keywords, which involves scrolling through a neverending list of alphabetically sorted categories:
There has to be a way to replicate this in WordPress.