In 2.2.1 we changed the oplog format to fix idempotency issues. We still have code in the server to handle old style oplog entries, it would be nice to get rid of that code now.

This could have implications for users who try to upgrade from some very old (before 2.2.1) version of mongodb up to 2.8. Even if they go through all intermediate versions (2.0->2.2->2.4->2.6->2.8), they will need to make sure that before going from 2.6 to 2.8 all nodes in the set have replicated past the last old style (pre 2.2.1) oplog entry, otherwise a 2.8 secondary will crash upon replicating an old style oplog entry.