Since the new talent system was introduce and MOP it has brought many changes to the impact your character. Some are minor changes some are major changes that have happened to all classes. With the Mists of Pandaria release everyone needs to be aware of the changes that took place. Here are a few things to look out for when it comes to your class

Death Knight
Presences have changed from what you use to know. Frost Presence now will increse your Runic Power generation, and each talent specialization to lean toward corresponding Presence for normal use. Necrotic strike was added as the new talent Death Siphon, however both cost a Death Rune. which means they can only be used when one of your runes have been transformed into Death.

Druid
Druids have had big changes happen to thier class that will give them a different look especially when it comes to PVP. The Feral specialization has been split into Feral and Guardian specializations. Each of the four specializations have a more individual and different set of tools, however all of the spells available to the specialization are now useful. For instance Feral and Guardian druids can cast Healing Touch for a decent amount of healing, and Balance and Restoration druids can make better use of the Cat Form and Bear Form abilities.

Hunter
Oh yeah!!!! There is no longer a minimum range for Hunters. The ranged weapon slot has been removed from the game; hunters now equip their ranged weapon in the main hand weapon slot. Hunters now benefit from Expertise as their ranged attacks can be dodged. Any type of pet can now choose any specialization.

Mage
Mages received lots of changes intended to change out rotations and make them more interesting. Arcane mages now have a more complex rotation. Fire mages now have tools to control the random nature of their spells. Frost mages are now considerably stronger at sustained damage dealing. Alot of the crowd control and survivability that was unique to Frost has been spread to the whole class, or made available to all specs as talents.

Paladin
Not to much difference here you can still faceroll this class. However, paladins now have several ranged abilities, though remain primarily melee combatants. In-line with streamlining raid buffs Auras have been removed which makes a big difference on this class, and the buff focus is now on Seals. Paladins can now store up to 5 Holy Power at a time, but still can only use up to 3 on each Holy Power spender.

Priest
Mana pools are fixed for all casters so they say. Chakras are now more similar to warrior stances. Changes have been made to Discipline to further support the damage absorption playstyle (you can actually play this spec with more ease than before and do well with it). Shadow priests now have Shadow Orbs as a resource, which are generated by some of their basic spells, and spent on either strong damage or crowd control.

Rogue
Finally, Poisons are now Spellbook abilities that affect both weapons at once. Poisons are divided into two categories, Lethal (Deadly or Wound Poison) and Non-Lethal (the rest), and you may have one poison of each type active at once. Many abilities that used to be restricted to a single specialization are now talents and available to all specializations; so you can have a Shadowstepping Combat rogue or an Assassination rogue with Cheat Death. Dagger speeds have been normalized such that the niche of the “fast dagger” intended for the off-hand no longer exists. Any dagger should be fine to equip in either hand.

Shaman
Totems no longer provide long-term buffs yes hacks are gone for shamans. Some of the old passive effects are now innately provided by the shaman. Totems instead have short-term situational benefits. In light of this the totem multidrop UI has been removed, though all totem spells are now sorted under draggable elemental flyouts in the Spellbook.

Warlock
The warlock specializations have a unique secondary resource which allows them temporary surges in power. Affliction warlocks focus more on layering multiple periodic damage effects, and amplifying their effects, and use Soul Shards rotationally for strong damage, or occasionally for utility as before. Demonology warlocks now have a new Demonic Fury resource, instead of Soul Shards, which is built by casting their normal spells. Destruction warlocks now use Burning Embers as a resource, which builts up slowly through casting their normal spells, and spent in bursts of massive damage. Warlocks no longer use armor spells, instead gaining additional armor and bonus health passively. All demons deal similar damage and are no longer linked to a specific spec. Doomguard, Infernal and Soulstone cooldowns reset upon wiping to a raid or dungeon boss.

Warrior
Rage has changed to function more like a valuable resource. Warriors no longer naturally gain Rage from taking damage; instead generate Rage with their primary attacks, some include Mortal Strike, Bloodthirst, and Shield Slam. Stances also now share a single action bar, and abilities are no longer restricted by stance. In addition, the benefits of the stances have changed considerably, with Berserker Stance allowing the warrior to gain some Rage from taking damage, Battle Stance making the warrior gain additional Rage from dealing damage, and Defensive Stance focused on survivability.