1. Titans. The pantheon paid special attention to Azeroth, and introduced a number of species indirectly with great potential.
2. Azeroth. Lots of magical things there, either by the Titans or naturally occurring on Azeroth itself. Magic mortals can use to gain great fame and power.
3. War. Lots of it. Hard times make hard men, and Azeroth has been in a hard time for quite a while. Even Lorewalker Cho comments on this in Pandaria, 'you've seen a lifetime of warfare haven't you?'
4. It's not as much as you think. This expansion has had more 'original content' than Pandaria, creating a whole new universe filled with backstory, characters, creatures, and places. We see a lot of familiar faces, true, but we also haven't been introduced to so many new characters for a while.