Upon the launch of MoP, I did something I've never done before at the beginning of an expansion. When WotLK came out, I went straight to leveling my Paladin main character from 70 to 80, putting off leveling a Death Knight for awhile. During Cataclysm I started out leveling my rogue from 80 to 83 but quickly returned to my Paladin, reaching level 85 soon after. Much later on I tried leveling a worgen and goblin.
When MoP came out, I did something completely different. I immediately began leveling a monk. After reaching level 75, only then did I finally begin my journey into the Jade Forest on my Paladin. Experiencing both the new leveling game as well as the expansion content gave me a pretty good feel of what's been greatly improved about World of Warcraft in this expansion.
AoE Looting – Time Is Money, Friend
Loot is everywhere in WoW. If you're a savvy goblin, you're most likely trying to accumulate as much loot as you can. There could be an epic on that corpse! Being able to loot more quickly saves a ton of time while questing and running dungeons alike. If you loot your corpses (which you should!) then the amount of time spent leveling is drastically reduced with this new feature.
Not Too Big, Not Too Small
Pandaria feels like it's just the right size for a continent. Northrend and Outland were too large and spread out. Pandaria has it all, without too much sprawl. Traveling around on a flying mount feels like less of a hassle, as any destination can be reached within a couple of minutes by air from the Shrine of Seven Stars. Compare that to Dalaran and Shattrath which often meant a long flight to Borean Tundra or Netherstorm respectively.
Variety, The Spice of Life
Upon reaching level 90, there were several new daily quest lines to explore, and new dungeons & scenarios. Having completed all of the quest lines except for the ones in Townlong Steppes and the Dread Wastes, I gained access to four daily quest hubs and an archaeology daily quest. All of the daily quests combined can be done in roughly 60-90 minutes, and the variety of different quests from day to day keeps it fresh. Cooking quests, fishing quests, farming, and first aid quests add even more variety to the "daily grind".
Portals, Portals Everywhere
The portal room in the Shrine of Seven Stars allows you access to anywhere in Azeroth. Finally we now have a portal to Dalaran! You can also access Shattrath and all of your home faction cities from here. There are no auction houses at the Shrine, but this is done by design: it keeps the Shrine a lot less crowded that way! Less crowd, less lag. Blizzard hit the nail on the head when they designed the Shrine.
Secondary Professions Aren't a Grind
Leveling cooking, fishing, first aid, and archaeology can be done painlessly over the course of completing daily quests. There's no need to spend hours leveling these professions the hard way, as the quests you are provided with will help you along quite a bit. Completing the fishing dailies in Jade Forest, Krasarang Wilds, and your home city will award bonus skill points. Ditto for the home city cooking quest. You can grow your own cooking ingredients on your farm in Halfhill, making the journey to 600 cooking very inexpensive. Archaeology was a cinch to level from 525 to 600; simply completing the archaeology daily quests a handful of times did the trick.