We heard from a few people either new or returning to WoW that the amount of information overload on our front page was reaching critical mass, so it occurred to us that it might not be obvious why the fabled Patch Day is such a ("Dogs and cats living together! Mass hysteria! Details at 10!") big deal in these here parts. Here's an older WoW Rookie on "What is a patch?" from Amanda Dean if you're new to the business of how Blizzard updates its content.
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!