I haven't had to approve any new themes on ss.org for over a year so some fresh designs would be cool.
As for your questions..
1) We don't keep stats on themes or ratings but from my dealings with the community alot of people actually use the paddy green theme - http://silverstripe.org/paddygreen/ terrafirma is also a popular one - http://silverstripe.org/terrafirma/.
2) As a bit of history - most of the themes on the site currently were designed / build by students as part of the GHOP program in 2008 it must have been. Not sure how many of those guys are still lurking round to offer support. Most I believe haven't stuck around. I haven't had to deal with any authors since the initial build and review so support I'm not sure.
Another theme gallery is http://themes.silverstriped.com/ which has mostly the same themes this site has.
3) As for features lacking, one of my *big* issues with the theme support is that you aren't able to easily bundle PHP code with themes so its very hard to bundle homepage layouts or specific widgets or anything else (like a custom login form). What I would like to see is the idea of 'sites' rather than themes as such. So a theme (or rather a created 'site') comes with a theme and its own set of PHP code for a customized setup.
But changing the current system is going to open up a whole nothing discussion :)