When adding categories to posts, it can be tedious searching the list. Having a live (not sure what to call it) search box, which would bring up the closest matches based off variety of criteria that you could stop typing at any time and select a number of categories.
For example: If I wanted to add the category "example" to my new blog post, and I were to use this requested feature, here is how it would work. I'd use the categories "add" field (which in this case would be more like a search field) and I’d start by typing my search of "example" (my demonstration assumes slow-mo is on ;))...
I'd type "e" and instantly the following results are displayed in the box below
ed
eddy
edit
emachine
examine
example
exasperate
expedition
explode
extradite
extreme
ex-wife
then you type the next letter in your search "x"
and it would further narrow your results to...
examine
example
exasperate
expedition
explode
extradite
extreme
ex-wife
then you type the next letter in your search "a"
and it would further narrow your results to...
examine
example
exasperate
then you type the next letter in your search "m"
and it would further narrow your results to...
examine
example
then you type the next letter in your search "p"
and it would further narrow your results to...
example
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The beauty of this is that you can stop at any point and add whichever categories come up. You'd likely only type the first few letters of whatever you want to add. Plus it'd help when you are trying to figure out which categories you already have.
I know it’s not a huge change, but it sure would give the overall feel of wordpress posting a much classier feel.