I recently attended a talk on Hadoop by Robert Chansler. One interesting concept I picked up was the concept of "federation" or simply "playing nice with users".
In the context of Hadoop, this means achieving backward compatibility with legacy versions of the software. E.g. when users run "old code" in clusters with a newer Hadoop version, their code can run correctly without modifications.
Titbit about Hadoop: the next Hadoop milestone will focus on security.