Also, 2.6 is not an all or nothing proposition that needs to be done in one release. Features with the most demand (whether it be the new write commands or aggregation framework improvements) will be done before others. We've done this before. When we released 1.0 that was based on 2.2, we also released hash based sharding with it which was a 2.4 feature. We did so because users demanded it.
As for pushing bug fixes upstream, we file bugs when we see them. Our VP of engineering was a winner in the MongoDB 2.6 bug hunt with SERVER-12878. SERVER-9848 and SERVER-14382 are among the bugs I've filed.