As of patch 2.3 the amount of XP earned per quest has drastically changed. You now get a lot more XP per quest AND the XP required per lvl is a lot less now. I find it hard to believe that you could lvl faster by doing anything other than quests since that patch came out.

Example. My solo lock started out in bloodelf starting area. Before i could finish all the quests in that area, they were grey already. Normally you'd go from to crossroads. Well, i skipped XR completely and went straight to tarren mill and form there to Ashenvale. Everything in Ashenvale is already Green to me. My lock is lvl 28 with a few hours playtime.

In ashenvale the lock teamed up with my 28 hunter, and they will dualbox from there on doing quests all the way

BBJ