Well I have my Drood up to 45 now, I kinda stuck with the mostly instance strat, but ended up wasting time running to too many different instances. I ran almost every instance, just so next time I boost it will be faster. here are my opinions on each instance.

RFC - not that great of EXP, but it will help you plow through 10-18

WC - there are good quests here, but I would skip it because it's too big and not friendly to aoe grinding(just too much running around for not enough exp).

SFK - Great for aoe grinding. If I had to do ti over again I would have skipped WC and gone SFK from 18-24

RFK - also a good aoe grinding instances good for 24-27. Lots of blues to DE, if you're into that.

SM - As many people have said the best for AOE grinding, and you could come here after SFK, but I like a little variety. Stay here til 40

RFD - I thought doing the quests for here would make up for the time it took, I was wrong. Skip it.

Ulda - I know it is a long walk, but now that you're 40 you can mount up and get there in 15 min or less. There are a ton of quests for it and I think it's really worth it. Plus the quests you can do just between Kargath and Ulda make it worth the trip. 2 times through + quests got me to 43

ZF - Pretty aoe friendly instance, plus all the quests are mostly in Gatgezan. Stay here til 46.

Note: it helps to have a mage and wowhead so he can port you to the differenet cities to grab the quests for each. Those quests can save a bunch of time.