(Ultimately, I would love to have a "promo page" where community maintainers can submit their community with a nice blurb and then it gets portrayed there for a week or so. But I haven't worked that out yet.)

It would be nice if such a page

* would be easy to find, like http://www.dreamwidth.org/communities (which doesn't have much information at this point),
* would show portrayed communities, like you suggested (maybe the maintainers of dw_community_promo could randomly choose three communities every week and write a short 'review' - given that community maintainers have the option to opt-in/opt-out,
* would make it easy for people to find all the official communities (and the popular ones which you also mentioned),
* would allow people to find/search communities by tag/category (list/tag cloud?),
* would make all the community-related information on DW Notes visible, with a link/text box/whatever,
*...

I'm sure I can think of more, but right now that's it :) Basically, what I'd like is an 'entry page' where all community relevant content is available from one single place.