For the uninitiated, Patch 2.4 is what is known as a "content patch," which is a sanitized version of saying "A ton of stuff gets added to the game and all hell breaks loose." This particular patch doesn't contain a lot of new content for people still leveling their first character, so you're unlikely to notice any differences unless you have a fairly high-level toon being affected by the class changes coming in or one who was beginning Karazhan attunement, which is no longer required. Apart from that, a rare new-5 man is being added (Magister's Terrace), a new endgame 25-man raid (Sunwell Plateau), and a host of new quests and a rep grind if you've got a toon at nearing 70.
The official patch notes are here, and WoW Insider has full coverage on the changes if you want a more in-depth look. Enjoy Patch Day!