First, go take a look at it.
OK, now that we have read it, what big changes do we see? As usual most clarifications were of the "I thought that's how everyone already played it because it was obvious" sort.
But there were two things that I think will change the way people play, and the sort of armies that they play. First, and least importantly, GW has shut down the stupid checkerboard tactic. This is where you deploy two interspersed units in a checkerboard formation, with the theory being that they grant each other a cover save even when they are both in the open. This was never something, in my experience, that happened all that much in practice, but I imagine the times when it was abused it really felt bad for the opponent. So kudos to GW for shutting it down.
Second, when a vehicle moves flat out and is destroyed subsequently, the passengers are removed as casualties. This is a big deal. This is a nerf to Dark Eldar, Eldar, IG, and some Blood Angels' builds. Basically, anyone who relies on fast transports to generate a cover save by moving flat out to provide some defense. I'm guessing this hurts the BA heavy flamer razorspam builds pretty badly, since Eldar vehicles are already hard enough to kill, and Dark Eldar benefit the least from moving flat out since they already get a 5++ save. But at very least, it gives those armies a pause for thought. Will you really be moving that DAVU Falcon all out early in the game?
Who does this hurt the most? Probably IG ships carrying melta/demo vets or stormtroopers. The Valks are AV12 but they draw a ton of fire and they rely on that 4+ save on turn one to ensure delivery of their payload. Now, if they rush forward all out turn one, they will probably lose their vets. If they don't they might not get their vets where they need to be in time. It's a tough position to be in. We'll see how this shakes out, but being a veteran aboard a Valk just became a lot more dangerous of a job to have.