Blizzard has been reconsidering what constitutes "a game sale" with StarCraft for quite some time now. They have been slowly carving out its mod platform, StarCraft Arcade, into a standalone, free product. They allow playing multiplayer with limitations, such as forcing free players to choose Terran (except for certain promotions). A few years in to StarCraft II's release, they even added "Spawning" to allow Starter and Wings of Liberty users to play locked content as long as a party member has purchased it, although Starter users are still locked to Terran.
Today's announcement is a little more conventional — Legacy of the Void will be a standalone expansion. You can purchase it without owning any earlier content. If you do own Wings of Liberty and/or Heart of the Swarm, then it will behave like an expansion, however.
The game itself will change significantly, too. At the competitive level, you often have a bit of a boring early game, unless one player decides to be a bit cheesy with their tactics. A lot of this is due to how long it takes to get from your initial six workers to being supply blocked. In Legacy of the Void, you start with 12 workers, twice as many as before. Also, each mineral patch has 33% less minerals, requiring bases to be taken more frequently and discouraging a maxed-out army from sitting on a handful of expansions to build a bank.
Many units were added and changed as well. Terran and Protoss are being pushed toward dropping units. The Warp Prism has its pickup range increased, to allow it to grab and reposition units from anywhere within a relatively large army ball, without needing to put the transport unit in danger. On the other hand, Terrans are able to pick up Seige Tanks while they are in Siege Mode. This allows a Terran player, who is paying close attention, to drop a tank for a quick, high-damage, and splashing shot, and then pick it up before it can be attacked. Siege Tanks have large range, slow rate of fire, and a relatively low health. If they are never shot at, though, while they're reloading their main cannon, then that nullifies their weakness, as long as you can keep the Medivac alive, too.
One thing that Blizzard disliked, however, seems to be Swarm Hosts. In Heart of the Swarm, competitions went on for hours, literally hours, as one component turtled in a corner of the map (or surrounded an opponent into a corner of the map) with free units. This was particularly problematic for Protoss, that has a highly efficient, ball-based army, and Zerg, which could counter with their own Swarm Hosts. Battles was commonly wave-after-wave of free units doing zero (or minimal) damage, ad-infinitum.
In Legacy of the Void, they do not spawn Locusts (free units) fast enough to pin someone down, or keep someone out, and these Locusts need to be spawned manually. Instead, they are intended as more of a sieging unit, capable of dropping free units into a base and walking away. They also do not burrow, unless that upgrade is acquired, which will make them easier to attack. On the other hand, the Locusts can fly to their target, where they must land to attack, as normal. The Swarm Hosts do not need to be in a dangerous location, just a potentially dangerous range. Whether Swarm Hosts, if they are upgraded with Burrow, can release Locusts while hidden is unclear. It is not something that I have seen yet. That said, the borrowed, space-control unit is now the Lurker, a Brood War alumnus.
Many other changes have been announced, but it always comes down to user testing.
As usual for a Blizzard title, no official release date has been given. A private beta will be "coming soon" to selected participants. It was also available to play at Blizzcon.