I would like to focus my efforts on the changes being made to the old content. (And all the little things that go along with that.)
A lot has been said and speculated about the new "stat squish". This is the part where your numbers are smaller. (Your health pool is smaller, the damage you deal is smaller, etc.) I won't ramble on about that here too much, aside to say that in a basic sense, imagine it as having the decimal point moved over one spot to the left. Nothing is "changing", just the presentation of the numbers are. The percentages are the same.
However, this does have a neat effect on our gear. In previous expansions, we have always exchanged our top end gear for the new expansion gear pretty much straight away. There were some exceptions (set bonuses, for example) - when you would see people reaching level 70 in The Burning Crusade, still wearing Tier 3 gear. (Or, certain Holy Paladins entering Naxx 10 in Wrath of the Lich King, wearing Tier 6 and BT gear.)
This is the first expansion where this will not be the case. The gear you obtain from Pandaria at level 90 will probably be with you for a while. Especially if you have obtained some Flex or Normal gear from Siege of Orgrimmar.
This change in gear is going to be retro-active. This means that they are taking the planer to the stats on all gear, across all previous expansions. Gear you obtain from Molten Core (For whatever reason) - will last you for a long time - in some cases - until you are 70 or even higher.
The difference in stats between Tier 3 and Tier 4 is less pronounced now.
Another subject related to the stat squish, which is often questioned - is your ability to solo old content. Blizzard has repeatedly stated that there will be no change here. If you could solo an instance or raid in Pandaria at level 90, you will still be able to do so once WoD launches.
While you will only be dealing "smaller numbers" worth of damage to mobs at level in Warlords, (such as a 20,000 aimed shot crit, or what have you) - you will still be doing copious amounts of damage to creatures that are much lower level than you.
The stat squish does not change your ability to obliterate the feeble weaklings below you.
I present to you, some things to do.
Another neat change, seems to be new animations and alerts to your procs. I know there are addons that add special notifications and more "visible" alerts and ques for when certain abilities are off cooldown or have proc'd, and it seems like Blizzard has made things a bit more visible for us.
You have a proc sir.
All in all, a lot of neat things are in the works, and we have much to look forward to with the coming launch of Warlords of Draenor. Stay tuned for more.