When I'm curious about which tags might be about to earn me another badge, it would be useful for the user profile to show the current "badge level" for the tags.
If I look at the bottom of the user profile where it's got tags and badges listed, the system clearly has all the information available - and I could just look through each tag and see whether I'd already got a badge for it or not. It would be simpler if the tags just showed the gold or silver badge icon "inline" though in the list of tags, just to the left of the tag name.
I've updated my Greasemonkey script to modify the blue rectangle to look like a badge (complete with gold/silver circle) for those tags that a badge has been awarded for.
To find out how close you are for other tags, you can click the triangle that's been added next to the tag to find the total votes for that tag (assuming you're not on the leaderboard for a tag in which case it's obviously simpler just to look there).
It'd go with the levelling up mindset behind the rep and the badges. You'd be able to see the levels and work out which one you need to cast the potion of alchemy on in order to go from silver to gold.
Sounds like a feature that would be useful for that single percent of users that are Jon Skeet.