Allows users with 20K on one site to have full privileges for everything except vote to close and vote to delete across all stack exchange sites on which they have at least 200 rep (100 for account association plus 100 earned locally).
Why? Certain skills are valid on all sites. Earning 20K rep anywhere shows that you understand on some level what makes a good question/answer and that you know how the general system works; you ought to be able to make good edits and flags at a new site right away. For tagging, you may not know aperture from f-stop on the photography site, but you'd know that '4.0' or '2010' probably aren't good tags anywhere, and you'd know how to fix them when you saw them.
The reason to hold back vote-to-open/close and vote-to-delete is that it takes a lot more involvement within each specific community to understand what's on-topic for that community.
This would have the pleasant side effect of helping seed new communities with privileged users faster as well.
As a side note, while I think this is a great idea, perhaps it's something to save for the 50K milestone.
An alternative that scales would be to divide rep requirements in half every time you double your rep at 20K and beyond.
Another alternative is to award an additional 100 rep for account association for each site on which you have at least 20K rep. This would encourage participation on more sites and help give 20K users an additional leg up on new sites without giving away the farm. So if you're 20K one one site and you associate your account to a new site, you get the base 100 for account association plus another 100 reward because you've earned 20K once: 200 total. If you're 20K on two sites, 300 total. The advantage here is that you wouldn't have to change the privilege calculation algorithm anywhere.