(yes I know, I just copied and reposted the same post from the General chat, shaman on me.
I can tell about my fresh experience. Started to level up 4 new drainei shamans, I already had one to 70.

From 1-10, no choice, I had to do "some" quests that didn't involve gathering quest items because it's such a pain to do it. Mostly was grinding mobs until I ding Lv. 10 while only playing the 4 shamans.

From 10-18, brought in the 70 shaman and started running Deadmines. Didn't loot anything because it wasn't really worth it and I really had some money to start with. I had them follow up from a distance while I fully cleared everyone zone (or almost full clear). I was getting about 75% of a level a run which is pretty damn got, it takes me like 15min to do one, that's over 2 level an hour. Just as a side note, I got from Lv. 8 to Lv. 35 in my week end.

From 18-25, Stockades is awesome. It's a small and very quick clear instance with only elite mobs which give much more exp. The only thing was that I would clear it too fast so that I had to wait at times, so then I would loot most of them to get a starting material base for one of my alts that's gonna be Enchanting/Tailoring. While I would wait on the reset I would log the others out and craft stuff to increase my prof or go play TF2. I would usually get about 30%+ per run.

From 25-35(current), I did one graveyard which is crap, don't go there. I did 2-3 Library to get the key and all. Then decided to start on Cathedral, which was the right thing to do. It's also a small instance with alot of mobs everywhere, just run around gather them all and kill em quick, get you thousands of exp real quick. I do a full clear on the instance all the time. I did loot for the first few times again to gather mats for my 2 prof and to a point I stoped and only looted the bosses for the shards of which I have like 60+ now not sure. Can't even seem to seel one of those things anymore... anyway...

I'm going to stay in Cath. until like 40, then go to Uldaman for a while. I skip Gnomeragan because it's a huge instances and the mobs are all spread out so it's not really worth going, better skip up to Cath where it's quick and all stacked together.

I currently sit at like 18 hours played at Lv. 35 for the shamans that weren't crafting.

So powerleveling instance vs. grinding quests? I'll let you figure out my answer. ^_^

Hope this helps,
-D