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Well nowadays, BC is free so the only xpacs you need to buy are WOTLK and Cata. There have been some major changes. First off, the Death Knight class was added in Wrath, Cata introduced a new stat called Mastery which buffs you based on your spec. Also, certain classes got new resources to use. Paladins got Holy Power, Warlocks got a new Soul Shard system, Death Knights use runes and runic power.

You'd have to buy original wow (unless you still have it), wotlk and cataclysm. The major changes are the new races (blood elf, draenei, worgen, goblin), new class (death knight). Everything is really simplified, like the talent trees. Cross-server dungeon finder, just queue and it finds you a group. All classes can heal. New spells...

Star Wars The Old Republic comes out mid Dec. get that...

You need wow , tbc , wrath of the lich king and cataclysm. Improved graphics, different ambient, new places, new raids and instances , new items, new classes and races , more quests. Hope i helped!