Please make the default in the addon directory to only show those themes, plugins, and applications compatible with the current release branch, and consider removing (or hiding from search and search engines) items altogether when the version of Vanilla they work with is no longer supported.
To elaborate more. There's no "vanilla 2.1" tag for the addon directory. There are lots of addons that are clearly depreciated (please bury them so that they aren't easily found when searching/browsing) , lots of addons that don't work for 2.1. If 2.1 isn't fully backwards compatible with 2.0, (and it isn't) it needs its own tag so that we can clearly see what does and does not work in the most logical place to look for that information - the addon directory itself.
Perhaps, what we need now is a list of plugins that are not compatible with 2.1, i.e. plugins that don't work and have NO fixes or tweaks supplied by forum members that make them work.
If at a later time a tweak or fix is found to make it compatible it could be moved back into the realm of the living.
e.g. these two plugins do NOT work in 2.1 and No One has supplied tweaks to make them work.
probably others as well.
Move them out into 2.0 only plugins (as sdaugherty suggests).
It certainly would be easier for the developers to determine what should be moved where if "members of the community helped here.... " Have you added to the spreadsheet yourself????
maybe anything not listed as working (and no fix provided) in the above discussion link (or spreadsheet) should just be moved to 2.0 archive plugins. and only after someone says something is working in 2.1 or a tweak to make it work, should it then be moved into the land of the living and breathing 2.1 plugins.
I may not provide the completed solution you might desire, but I do try to provide honest suggestions to help you solve your issue.