Just from a performance perspective, when your feed is being retrieved, I can only presume that the more communities you're subscribed too the more intensive the query is?
At some point does it not get prohibitive? Is there a limit on the number of communites one can subscribe to.
If true, I see this as a good reason for user created multi or meta-communities. This way, I can look at a feed of only a subset of communities at a time which should lessen the load on the server, and also be a better interface because I honestly don't want all of the communities I'm interested in being fed into a single feed.