1. It's the end of the expansion and despite the latest numbers (which people seem to ignore include the holiday bump) people are going to be dropping like flies due to attrition.
2. It's also the time of the expansion when people are most angsty towards returning players; "What, you mean you haven't done these heroics a million times? Vote to kick this noob!". Now also add the same level of vitriol in LFR and Flex raiding modes.
2a. Also consider that if you get to 90 and try to run heroics, you're not going to even remotely have any degree of fun because everyone will so overgear them that you won't even get a skill off before the tank is two pulls ahead of you. Or even pulling the entire room to the boss.
3. The story has been completely altered by all of the previous content patches to the point where it isn't nearly as impactful as it was earlier in the lifespan of the expansion. The Vale gates opening post-5.4 is a great example; The quest is still the same but the dialog has changed, completely losing its meaning from the original story line and almost makes no sense for a fresh, new-to-pandaria, player/character.
4. When you reach 90, you're going to realize that you're doing everything solo and you're so far behind the curve that your only solution is to KEEP doing things solo. Solo Timeless Isle grinding. Solo Heroic dungeon and LFR queues for gear and valor. Solo BGs for honor.
Coming back now would be a waste of money and time. You've waited this long, you're better off following the development of WoD and saving your money for now. If WoD appeals to you, come back shortly (like a week or two) before it drops to get your characters and game client in order and then use your 90 boost to get right into WoD's content to be on the same plane as everyone else.