I've found the best way to level alts is Pet Battling over by Halfhill in Pandaria. My Nightborne Priest I got to 90 that way (the rest was WoD Treasures, then regular leveling post 100), my Mag'har Shaman I'm at 61 with 16 hours or so played. Having a second monitor to zone out and watch Netflix or whatever helps. I think it's the fastest way to get to 90 personally. I really dislike the leveling changes so I just Pet Battle now.
I've been having what is apparently a scaling issue caused by inflated secondary stats on some Wrath of the Lich King items. In particular, my Highmountain tauren warrior leveled through Northrend and picked up a green, iLvl 87, 2h sword. All of the secondaries were in the +25 to +30 range, and the Dugi Gear Advisor decided it was better than the heirloom 2h axe I was using. So I went ahead and equipped the sword.
Drustvar is far and away the hardest Alliance zone, which is exactly why you want to do it first. If you have legendary items, they’ll still be usable, and if you don’t, this zone will still be easier early on than it will be later. Drustvar is also extremely fast and will let you jump out ahead of your competition in other zones, which means fewer players around to take your quest objectives.