Are we sure our alts are going to be able to fly in Draenor from lvl 90 and not 100?
The blue post that I can't find and link to since I am at work certainly phrases it the way that we are going to be flying at 90+.
This is huge for leveling your alts.
The XP going from 90->100 is about 8 million give or take.
There are at least 200+ treasures to pick up that give between 20 and 33 k XP, I do think this is with full heirlooms.
At 26,5 k XP on average for 200 treasures, they alone give 5,3 million XP, enough to bring you close to lvl 97.
Picking up garrison followers is also going to be easier, if they are not bound to quest lines.
I am thinking specifically of followers like Image of Archmage Varggoth, Abu'gar and Goldmane the skinner. You can just fly around and collect the items needed for these followers. I think getting the followers also awards you XP.
Choosing the woodcutting machine in Gorgrond will also net you a lot of XP, maybe 500k? plus a lot of garrison resources.
To top it all off you can easily fly to all bonus objectives for quick and easy XP.
The only quest line I would feel to be mandatory would be the Spires of Arak chain for the salvage yard. Even that is not so mandatory anymore now that you can just buy 645 ilvl follower items for 500 G each.
Getting to all those garrison resources in Nagrand that required gliders is going to be really easy. I haven't even bothered with most of them on my main.
Farming Lumber if you have a lumber mill will also be stupidly easy.
I've got about 15 alts between 85 and 88 so this obviously has me excited to try it out.