It is really simple to add a search criterion by version number since this information is available on WordPress Web site.
Oh, no that's the EASY part. The hard part is making sure that the data is CORRECT. See, that field is available insofar as the developer updates it. Lord knows my plugins don't all say they're 3.1.1 compat, and yet they are.
Yes this will penalize those plugins who have not been updated even if they do work with the newer version of WordPress but then again since we could search by plugin version number that should not represent a problem.
Right. That, in a nutshell, is why this is more complicated. There's no reason to update for every minor iteration sometimes, and to have my perfectly valid plugin excluded just because there were no changes from 3.1 to 3.1.1 strikes me as a very bad idea.
Basically you're amping up the workload for developers who would rather be making magic than filling in paperwork.
Forgot to mention that the existing rating system is also there to evaluate if the plugin work or not.
That one's pretty new, but the problem with THAT is it's user-driven, and not everyone marks it as works/doesn't work. I'd be willing to bet that more people vote 'doesn't work' actually, in an inaccurate ratio, since people who have issues are more likely to go and mention it.
It would also be possible to add a search criterion by the number of download.
I'm not sure that would help anything other than popularity, which isn't a measure of anything useful, code wise.