Zarhym has posted information about an interesting change in effect on the PTR right now. Blizzard is tweaking the way that character nameplates (those names that you see floating above everyone's heads) works. First, nameplate vision has been extended, so that you can see player names from farther away. They've also adjusted some line-of-sight issues, and players will no longer be able to see nameplates through anything that blocks line-of-sight (so no nameplates of players hiding behind doors or walls). And multiple nameplates in one space will now overlap -- we'll have to see exactly what this looks like on the PTRs, but it sounds like a big group of the same mobs (i.e. Onyxia whelps) will now just have one nameplate to share.
Interesting stuff (and I didn't realize that some of this stuff, especially the nameplate overlapping, was even possible). Zarhym has a few other notes in the thread about how you can change or update the options on the PTR, including a checkbox to turn on or off totem or pet nameplates. Elsewhere, Ghostcrawler says that it's not just line-of-sight, as Arena pillars and other "objects" won't affect the nameplate view, but doors and walls will. If you're on the PTR, keep an eye out for these changes, and note that Blizzard is still asking for feedback. It's unknown which of these changes (or others) will end up on the live realms in the future.