If you aim for a painless levelling, I would tend to say: get to level 15, run instances until you're 30, go to Dustwallow marsh because it is such a mine of easy quests, clean the zone while queuing for instances. Queue instances non stop until level 58, because 40 to 58 is more of a pain than anything else.
At 58 go to hellfire peninsula and quest your staff off while being queued for dungeons all the time. At 68 move to Northrend, preferably Fjord, while, guess what? Queuing dungeons. :]