Mark Rosewater hyped up the article he had written that was published on the mothership today pretty highly. And wow, was he ever right to do so.
Whenever an article starts, "Magic is, at its core, a game about change," it's natural for the entire playerbase to sit down and think, "oh boy, here we go," preparing for the inevitable reactionary backlash, but the change he announced to the block system in today's article, appropriately called Metamorphosis, has been overwhelmingly well-received, and a sign of the maturation of the game.
In essence: Tarkir will be the last block as we know them, with three sets (first one large, second two usually each small but of various sizes throughout the years) in the fall, winter, and spring. Instead we are now getting two blocks of two sets each, first in the fall and winter, then in the spring and summer. What's being lost as a result? Nothing more or less than core sets - Magic 2016 will be the last of them.
The reasons for this are many and varied. Attempts to do a third set that coherently wraps up the themes of the first two without feeling redundant and while also being original (i.e., to solve the "third set problem") have been misses more than they have been hits. Core sets have to appeal to two different audiences (beginning and experienced players) in two different directions, and rarely truly succeed at either one, let alone both. And a one-block-per-year system means the storyline progresses at a snail's pace.
This has implications for Standard too - the format will now contain three blocks at a time instead of two, and sets will rotate after eighteen rather than twenty-four months. All is laid out in handy visual form in the announcement article.
None of this affects supplemental products - we will still get our summer supplements and Commander precons. Turns out, this was the one success of Lorwyn-Shadowmoor block that truly needed to be brought back!