ok what u need is to be a group of 2-3 ppl 1 tank 1 healer and 1 dps (can do it without dps)
So last night i did Nexus with my lvl 72 DK and i am blood specc so iam not tanking and in the group i had a lvl 70 healer and a lvl 72 hunter and i got from 50% to 100% on the xp bar under less then 30 min, u can do allmost all the bosses some is harder than other, for an example the big stone guy is hard when he goes frenzy. and all u need 2 do when u have done the instance is go out and reset.
hope this helps u with your leveling /smile
Cheers Desus