The guide mentions pretty much all of the significant changes that have occurred in the last two or three years -- from the talent system overhaul to the addition of battle pets and transmogrification. The guide also serves as a nice one-stop-shop for links to other, more detailed, guides, walkthroughs, and FAQs, so it's a nice hub from an organizational perspective, too, even if you don't need all the guides.
If you are indeed taking the plunge again after an extended absence, welcome back, and along with Nethaera's guide, you may want to check out some of WoW Insider's as well!
- Seven things every newbie needs to know
- Five reasons patch 5.2 will pull you back into WoW
- Everything you need to know about getting started with addons
- How to make gold without breaking (much of) a sweat
- How to get started with transmogrification
- How to get started building your own battle pet arsenal