If you love running old content for transmog items, you'll love the new legacy loot system. Whenever a player enters a dungeon or raid where they are 10 or more levels above the content, the dungeon or raid bosses drop an amount of loot equivalent to what would have dropped for a full group when the content was current. That means players could earn loot for five people in dungeons and loot for 20 people after soloing a raid. The legacy loot system also drops other gear types, like cloth dropping for a plate wearer. It's now much easier to get the transmog gear you want, and you can also collect gear for your alts on the same run.
For reference, my previous best time abusing Method-0 in 7.3.5 was 32.4 hours(as show in the old thread). A difference of just over 8 hours; which isn't all that bad, all things considered. And I believe I made several errors along the way, which could be cleaned up in future runs. I believe that that gap could be significantly closed by using a monk's bonus, but I hate playing monk...so...yeah. YMMV
Bags: This SHOULD be a no-brainer. The cheap-o version are Netherweave bags, but if you have the gold I recommend you get the largest bags you can afford, since it means you won't have to stop to clean them out as often. In fact, if you have an excess of gold, it's probably not even worth your time to stop and loot mobs that aren't quest objectives. This seems like a small thing, but over the course of 100+ levels, all those clicks to loot add up.
When you have completed your third zone, you should be nearly max level, but you still have work to do! Head back to your faction hub and complete the last foothold quest to unlock your fourth mission table champion. After, you will want to do your newly available invasion missions for your faction. This quest chain is one that will be time gated by reputation levels earned with the 7th Legion for the Alliance and iThe Honorbound for the Horde. Complete as many quests as you can and then finish up your leveling with side quests you may have left in previous zones as necessary.
Take breaks. Unless you're straight up no-lifing it over a weekend or something, this leveling process is going to take ~40 hours. But any time you do need to step away, log out. That tiny little bit of rested exp is insignificant...but you never know when something might come up and you'll have to leave for a longer period of time than you expected. Better to be logged out and safe(especially with War Mode on) than AFK waiting for a d/c. Plus, you'll need to stretch, eat, shower, and maybe sleep. XD
New to Patch 8.1 is a new leveling flask called the Draught of Ten Lands which increases all primary and secondary stats by 20 and experience gained by 10%. This effect lasts for one hour and persists through death, similar to other battle flasks. This item can be purchased using a new Patch 8.1 currency called 7th Legion Service Medal or Honorbound Service Medal which can be earned doing the new incursions and Warfront activities. Simple earn these medals on your main character, purchase the flasks, and send them to your other characters. The cost is 10 of the medals.
Darkmoon Faire: The Darkmoon Faire comes around once a month and provides two ways of bonus XP. You can ride the carousel to get the “Whee!” buff (10% bonus XP for one hour). You can also get the Darkmoon Top Hat by playing a few games, which gives you 10% bonus XP but only works during the Darkmoon Faire. Both of these are generally not worth it for the most part, as you spend more time running to get the buff than you gain in XP when leveling.
I've identified a pesky bug that had me confused for months now. I would get serious slowdowns (from 60fps to as low as 20fps) whenever the dugi waypoint arrow was active and I started moving across the map (especially quickly like flying on mounts/flying masters etc.). I tried everything, and all that could fix it was clicking "Remove all waypoints", so that the arrow turned off. I thought maybe the arrow needed to constantly update locations/routes, and a new game update had perhaps broken that feature a bit.
The Horde will now control most of Kalimdor, and the Alliance most of the Eastern Kingdoms. For a low-level character the quests and storylines won't change, but for high-level players many zones will see changes via phasing. Teldrassil is burned and Undercity is in ruins as the story progresses by phasing. At max level Arathi Highlands and Darkshore are phased to be updated to reflect their status as locations for Warfronts.
If you queue as a healer - take care to always have a stack of water of appropriate level. You can buy it at an innkeeper for next to nothing, but remember that it will become too low-lvl for you very fast, so you need to buy new water every 10-20 levels. Alternatively, you can buy a stack of every tier of water in Stormwind, and just always carry it with you. Another alternative, if you see a mage - always ask him (politely) to create a table for you. Grab 2-3 stacks so you have extra for the next runs.