This is by no means the way to go if you want to maximize your experience in the game, as this method will bypass almost all the content in the game and essentially ignore many of the fun, unique aspects of WoW. This is not recommended for new players and is meant for players who already have experienced the content and just want to level up an alt to play with your friends.
1-20: Newbie lands, stick to the friendly zones. Just grind. The quests are a waste of time up to level 20 if you are focusing on maxing out xp per hour.
20-26: Wetlands quests and raptors/slime/orcs for grinding. Redridge mountain Lakeshire quests.
26-30: Duskwood quests and undead for grinding. Hillsbrad critter roamers for grinding.
30-35: Did these all in a day on Daggerspines in Hillsbrad right next to Southshore along the coast. Huge spawn, very fast respawn rate, easy mobs to kill. Purgation isle (island in the very far Southwest water area of Hillsbrad) also a nice secluded place you can grind on undead.
35-40: Alterac ogres for grinding.
36-39: Drywhisker kobolds in Arathi east of Hammerfall for grinding.
36-40: Hillsbrad southshore quests.
38-40: Desolace quests and kodo grinding.
37-41: Cresting exiles at the circle of outer binding in Arathi also extremely easy mobs with a fast respawn.
41-45: Hinterlands trolls, owl beasts, and wolves for grinding and quests. Badlands ogres, gnolls, and quests.
45-48: Tanaris quests and pirates on the eastern peninsula for grinding. Stranglethorn vale quests. Badlands greater elementals for grinding.
48-52: Felwood deadwood gnolls for grinding and quests from the sanctuary in the south. Blasted lands dreadmauls for grinding. Un'goro crater (best started at 50) for primarily quests and grinding off the plants and tar elementals. Azshara undead highbornes and thunderhead hippogriffs for grinding.
52-55: Azshara blood elves for grinding. Burning steppes dreadmaul rock ogre caves firegut ogres for grinding. Felwood irontree woods and cave for grinding. Western plaguelands questing for argent dawn and grinding on undead throughout the various camps.
55-58: Eastern plaguelands quests from the sanctuary, grinding on undead at the ruined towns. Winterspring grinding at winterfall village or Lake Kel'Theril undead highbornes. Blackrock Stronghold in Burning Steppes for grinding.
58-60: The yeti cave in Winterspring, due Southeast of Everlook. Grosh Gok compound ogres in Deadwind pass. Eastern plaguelands Fungal Vale undead grinding and argent dawn token farming. Moonowls in northeast winterspring for grinding.
60+: Outlands, questing and grinding.