In its current beta form, a Warlock can summon a Demonic Circle that will stick around for six minutes, and teleport back to it from 40 yards out. This is like clicking on a "Home" button on a webpage: as long as you're within the range of the circle, you can always go "home".
It is still unclear whether the teleport effect is affected by line-of-sight, but my guess is that it will be subject to the same limitations as the mages' Blink. Another downside is the possibility of losing our pets as we plant the Demonic Circle.
Creative warlocks are already thinking about neat applications for this ability: using it to zip around in the Arenas, a quick recall button for feinting maneuvers in the battlegrounds, or simply to shorten that run between the mailbox and auction house.
Most importantly, Demonic Circle is a big step up in our "escape tech" department. For the longest time since patch 1.8, Warlocks have been relying on Death Coil (with Fear and Howl of Terror, to a smaller extent) to create that space between themselves and their melee banes: rogues and warriors.
While the fear/ horror effect is still a much-maligned ability for some classes, its defensive utility has become more limited against the very same melee classes that warlocks need to stay away from. This somewhat makes warlocks a "win-more" PvP class, with very little chance of turning the tables around by playing defensively if we're caught in a bad matchup or situation.
Much of our PvP and Arena viability stems from not being shut down too quickly by rogues, warriors and other melee threats. Unfortunately, they have been doing just that, even with the SL/SL tough-it-out spec and stacking for Stamina and Resilience. Warlocks are pretty much toast or "mushrooms", when we're caught by melee DPSers.
Warlock blogger, Jagoex*, looked at the Warlock vs melee issue and made an interesting case against Resilience. According to him, warlocks have little chance against melee classes because Resilience counters all aspects of our offensive power, namely crits, DoTs and drains. In other words, in the current state of the game where every opponent is stacking resilience, warlocks are simply not putting out enough damage to have a fighting chance against the auto-swing damage of melee DPSers.
Demonic Circle gives warlocks the ability to get out of sticky situations, but it could also get us into stickier situations, since our teleport destination is visible to all. Consistent with the "power with consequences" flavor of warlocks in general, it is certainly not a get-out-of-jail-free card.
We should see some interesting team plays against warlocks. I can just imagine a stealthed Rogue waiting at the circle, while his Warrior friend hustles the unsuspecting Warlock.
What about the PvE applications of Demonic Circle? I'll expect this ability to come in useful in certain encounters, where warlocks can set up a safe spot to get to in a jiffy. A couple of raid encounters come to mind: Archimonde, for example. Demonic Circle will be handy to get away quickly from the Doomfire trails, or to avoid the fall damage from Air Burst more reliably, if mid-air teleportation works.
Other than longer encounters, I don't see warlocks dropping circles with every trash pull, especially if it involves giving up our pets. A secondary effect that has been suggested earlier by Kalgan for Demonic Circle was to allow quicker pet summons if the Warlock is standing within the circle, akin to the effect of Fel Domination. We shall see if this gets into the final build of Wrath.
With the homogenization of cloth gear in the expansion, warlocks may lose the passive survivability provided by a class-defining stat: Stamina. Warlock gear has traditionally been heavy on Stamina; will sharing the same gear make warlocks as squishy as priests and mages?
Priests, and particularly mages, have compensated for their lack of Stamina with their range of defensive abilities. The 12k-life question: will Demonic Circle be good enough to do that for Warlocks?
*Jagoex is apparently another recent victim of account hacking; I hope everything's well. Dear readers, please safeguard your accounts!
V'Ming thinks Archimonde is secretly a tree-hugger. He gets really pissed when we disturb his quiet time with the big tree.