Huge changes went into place for the Mists of Pandaria Talent Calculator, causing a huge storm of community posts about the various changes. One thing that is very clear is that Blizzard is really trying to perfect the idea of playstyle choice. Not just in terms of class specialization, but in the various tools and abilities that are now available for an entire class via the new talent system.
Mages were pretty excited during BlizzCon to see a talent tier dedicated entirely to affecting their main crowd control effect, Polymorph. However, that has now been replaced as a tier that gives more options and style to how a player regenerates their mana. Other major changes were visible in the Paladin's talent tree. Talents like Holy Prisim, can be cast on enemies or allies with varying AoE effects if it's cast on friend or foe.
But talents weren't the only thing getting changed. Even class abilities are being adjusted and even consolidated. For example, tanks now have a debuff called Weakened Blows, which weakens a target's physical damage by 10%. This debuff is applied when a normal rotation ability like Thrash, Thunderclap, and Hammer of the Righteous is cast. This helps eliminate the incursion of a global cooldown on abilities that felt less interesting and helps ensure the maximum amount of debuffs are up on a target.
These changes definitely are going to bring an entire new learning curve for the entire player base. The best recommendation is to familiarize yourself with the changes coming to your class and be ready to experiment when the beta finally launches.