I dont believe it is too crowded... Although there are a lot of feeds going in - I dont often see anything irrelevant and I find it quite interested to see what others are writing about.

I have the DrupalPlanet feed aggregated as one of the feeds for my Google Reader.

.. others filtering, hightlighting or providing alternatives. None of that has happened, so I guess it is not too crowded.
I do, however, think it wise to filter out the feeds now and again to see if what people write is really about Drupal. Some feeds contain 1 in 10 posts about Drupal. The other nine being about personal stuff, or other non-Drual-related matters.

I know abuse and moderation adds to the workload of those managing it - but maybe there could be a way for people to report feeds?

Maybe voting could help?

I agree with Ber about filtering out feeds now and again. Some of those feeds might even point to 404's now! I noticed one the other day where the 'read more' links did not point to the same article the feed to was referring to!

Another possibility is to "force" people to only have their drupal tag feed's aggregated (as I do for TMJ).

I don't think it's too crowded but I do feel the list could use a clean-up. For instance remove all sites that haven't posted something in 2007 and remove all dead links.

.. others filtering, hightlighting or providing alternatives. None of that has happened, so I guess it is not too crowded.

This is a fallacy; it just depends on one's internal treshold to the crapfest (mollom anyone?) on Planet.

So, just to prove you wrong :

http://heine.familiedeelstra.com/planet-drupal-highlights