One of the problems with leveling through open world content pre-WoD is that any quest that involves collecting something you have to click on disappears when one character clicks on it. So quests like "Collect 8 mushrooms" become "Collect 40 mushrooms" and, even worse, your characters are all competing with each other for spawns. However, if you can pull it off with the right chain in the right zone, questing can be faster than dungeon running, slightly dependent on which dungeons you end up running.

With this in mind, my current strategy is:

1-20: Quests, rare spawns. Tirisfal Glades is the best zone if horde, not sure for alliance.

20-40: Dungeon spam. Tip: If you are only leveling four characters, have one character stop experience at level 15. This means that the only two dungeons you can get on the group finder are RFC and Deadmines. Deadmines is okay, RFC is excellent. If you stop at 20, you include Stockades (great!), but you include SFK (meh) and Wailing Caverns (Argh!). If you do this, you can do this all the way to 60 as you don't have to deal with the very long, slow and often challenging dungeons that pop up from 40-60.

40-60: Questing. Northern Stranglethorn -> Felwood -> Winterspring. Particularly on the horde side, Northern Stranglethorn has no collect quests where you click objects and the beautiful Nesingwary chain. For Felwood, you skip the quests associated with the Nightelf town in the north. You don't skip a collect quest to pick up a bunch of weeds before Shadow Hold, as Shadow Hold is very lucrative. Winterspring is the second best questing zone in the game, though I skip the yeti chain.

60-90: Dungeons. I'm unaware of any tricks or any decent zones in this level range. Running BC Heroics used to be excellent, but when they changed the way dungeons award XP it became awful. This level range is not any where near as bad, dungeon running, as the outrage a month ago suggested it was, but its still the biggest slog in the leveling process. Seriously, if anybody has any ideas, I'm all ears.

90-100: Treasures and Bonus objectives. Much faster with flying, but still probably the fastest strategy if not. Spend garrison resources on the XP pots. Make sure that, when you loot treasures, you do it one at a time and fully let the character looting the treasure pick up the item and close the loot window before the next character loots, otherwise the second character does NOT get XP, but the treasure is still permanently looted. Just questing is also not slow, as collection spawns now begin spawning per character.

100-110: If you are patient, invasions. Invasions give roughly one and a bit levels for doing all 6 quests. Also, even if you are patient its worth doing Val'sharah which is the best levelling zone in the game if you have flying. Dungeon running is not slow if you are set up for it, particularly if you can get a hold of a useful legendary through the BtA Wakening Essence random legendary from an alt or other twink gear. As I'm doing invasions/Val'sharah, I queue into random dungeons and complete them if I end up rolling something quick, and leaving and going on cooldown for 15 minutes if I roll something slow (Halls of Valor!). Otherwise, questing is reasonably quick, as long as you stay well away from Stormheim.

110-120: Early days, and some folks probably have better ideas than me. I seem to average about 300000 - 400000 XP/hr doing dungeons and 400000-600000 XP/hr doing quests, though that slows down considerably at certain points. Certainly feels like a bit of a slog at the moment, but that might just be because of no heirlooms.