I just asked a question over on Ask Different, and when I got to the tag selection stage, I found a little dollop of UX goodness waiting for me:
The system seems to have picked out likely keywords from my post and is helping me, a new user of the site, correctly categorize my question. Clicking on any of these adds it to the tag field, also a nice touch.
The same feature does not seem to be present on Stack Overflow (tested both with my real account and a dummy new account created for this purpose):
Interestingly, this has been requested quite a while ago and declined: Auto-tagging (also When asking a question, could the page suggest relevant tags? and Have Stack Overflow automatically suggest tags) but it's apparently now been implemented.
This seems like a great thing to have for new users (and I see it's even here on Meta!); can it be added to Stack Overflow?