R&D head Mark Rosewater is probably the single person with the most Magic design knowledge and experience on the planet, since he's been leading set designs since Tempest block. However, he is also only human, and can only work on so many things at one time. Thus, there has been an apparent shift of responsibilities in the R&D Pit - Rosewater will now be the lead designer for every fall large set that kicks off a block, and not for anything else.
When Tumblr user tyrann13 asked Mark for clarification on his question-and-answer blog - "If you are now leading all large fall sets going forward, does that mean you will no longer lead small sets? Or would you do back-to-back designs?" - he answered:
For the near future I will not be leading winter or spring sets.
Since so much of how winter and spring sets are designed depends on what happens in the fall, and since he will still apparently be on design teams for small sets in a subsidiary role, Rosewater is not going anywhere any time soon. However, what may be of note is that this coincides with a block with "unique block structure" beginning in fall 2014 - which may have Mark leading multiple sets in a block for the last time, and leaving his indelible touch on those cards.