Legion, which looks like it could be the best expansion since Wrath of the Lich King (or even surpass it) is going live in the United States in a mere seven hours (it might be just one hour in Europe.) This is mostly a cause for celebration - we get to go get our artifact weapons and start fighting in the Broken Isles (assuming that DDOS attacks - which I guarantee you are going to happen - and the overall lag of having everyone in Dalaran at the same time - though thankfully only briefly - aren't going to make the game unplayable.)
But there are some things that are going away. These are:
The Demonic Invasions:
If you've decided to level up new alts as fast as possible, the demon invasions have been an amazing opportunity for that - even at high levels you can get about a level per invasion (and at lower levels you can get several.
If you really want to get some alt up to 100 for Legion and not burn your character boost on him or her, you'll want to do these invasions non-stop until the expansion launches.
Additionally, I'd assume we're going to lose the Illidari camps in the various zones (not sure about the ones in Stormwind and Orgrimmar,) so if you have Nethershards, make sure to get your Fel Bat Pup and any Crystallized Fel you can to upgrade your weapons (though this latter part is almost totally moot, as you'll be getting your artifact weapon before doing anything else in the expansion.) If you haven't gotten gear in all armor slots for all armor types (and don't forget the cloak!) now's a good time to spend your shards on those cosmetic items - note that if you have gotten all the pieces of a given armor type, you won't need these cosmetic sets.
The Grove Warden, like the wolf mount from Siege of Orgrimmar, is going away after the next expansion launches. The Grove Warden is a very cool mount, and while there are other moose mounts coming in Legion, they don't have all the bells and whistles that the Grove Warden has.
At this point, you'll probably need to drop a substantial chunk of gold to get a run unless you're very lucky. Beat Archimonde in Hellfire Citadel on either Heroic or Mythic and you'll get the item that begins the quest (just to be safe I'd try to complete the quest tonight as well.)
Guaranteed Felsteel Annihilator:
This is also the last time to get a guaranteed drop of the Felsteel Annihilator mount from Mythic Archimonde. This mount is not going to go away, but it will not be a guaranteed drop.
And that's about it!
The Broken Shore event will remain as the introductory quest for the expansion, though once Legion has launched, if you've done the event on one character you'll be able to skip it on others. You can apparently start these quests at 98, meaning that I'd assume after the Broken Shore, Ulduar, and Karazhan, you'll probably hit level 100. So no rush on this!
Expect a huge rush when the expansion launches, and tons of lag in Dalaran both over Deadwind Pass and the Broken Isles. Thankfully, you won't be staying there long, as players will split up by twelve and then thirty-six as we all go to pursue our artifacts, though you can expect DPS artifacts to be more "crowded" quest areas. Thankfully, the climax of each artifact quest is a solo-scenario, so you unless there's a problem with instance servers (and there might be) you shouldn't have to worry too much.
Oh, and get rid of any garrison items you don't want to have taking up your inventory. We're leaving those places behind. Also not a bad idea to clear out gear from your bank and bags - with the new transmog system, you'll make a ton of room from things that are now saved to your appearance tab.