Aside from any impracticality that could arise in implementation, I like the idea of federated communities between servers. I mean why not extend the possibilities of federation even further? Community mods or users could de/federate from communities on other servers with the same names or core themes should they so choose. In consideration of difficulties with moderating spam and other materials from other communities generated with the same name, I think it makes sense for that kind of community federation to be opt-in rather than opt-out.

If it goes the Reddit route, one of those communities will definitely border on dead and the risk for moderators/servers having too much power/influence within the larger communities continues.