bloated and overengineered just like kde4. Can't you really come up with something lighter? see kde-look.org for how it's done.

Filters? How can I sort "Done" by popularity without showing others?

The first sentence wasn't really necessary, was it? You can try to disable AJAX (there's a link at the bottom of the page in Brainstorm) and see if you like it better.

As for your second question, it's not possible at the moment (as far as I know). I proposed a few changes to Brainstorm some time ago which involved categories, sorting and filters, but someone also has to do it. If you would like to help out you can talk to us in #kde-forum on freenode or write a reply here.