First of all, Metamorphosis will no longer replace a summoned demon -- a no-brainer feature that should have been there from the start considering that most talents in the tree buff up the Warlock's demon! It will also now increase armor by 600%, up from 360%, a coefficient that scales better with a Warlock's base armor of... cloth. Significantly, the demon form will now also increases all damage by 40%, after players realized that turning into a demon with many melee abilities and being cut off from normal spells actually reduced their DPS. Speaking of normal spells, Warlocks in demon form can now cast those, too. These changes come in addition to a few others that Blizzard is working on such as a few demon form-exclusive abilities -- and it seems like Demonology will finally be truly, diabolically fun. [EDIT: In my excitement I forgot to mention that the spell was reverted to a 5-minute cooldown and the form lasts 45 seconds. Still awesome!]
I griped about this on the last Blood Pact along with a throng of other Warlocks, and it seems like Blizzard has heard us -- Corruption will now be baseline instant cast! It's a change that has been a long time coming, and it's finally turning real. Improved Corruption will instead increase the damage of Corruption by up to 20%, effectively negating Empowered Corruption and hopefully freeing up space for a new, exciting talent or at least more points to use in an already bloated tree.
Lastly, the highly desirable DPS talent Ruin will be moved higher in the trees, making it more accessible to other specs. It will take the place of Devastation and become a 5-point talent to grant the same bonus while Devastation will move deeper into the tree and become a 1-point talent for 5% spell crit. This change allows players to take Ruin without having to sacrifice their 51st talent point in another tree. Koraa also mentions that Pandemic animation and some of its spell mechanics will change. I'm quite certain these aren't everything that will change for Warlocks this build... will we finally see Fire and Brimstone and Chaos Bolt in action?