There is just WAY too much down time in Classic. Im level 37 and i find myself spending most of my time running between zones and not really "playing" much. I liked the activity before, but it just stops me from playing for long periods and it made me take a long break last week. Its just dranning to not have objectives or objectives that are not close.
I actually started leveling in Retail and while it does have its own problems, such as deadzones, where levels give you nothing, but i do find it a bit more enjoyable to put 3+ hours into at the time.