I started World of Warcraft as an Alliance player, then I moved on Horde for a long while and after the release of Cataclysm, I re-rolled Alliance once more. But things had changed, the Alliance leveling areas were not the same, therefore I realized I had to follow a totally different leveling path. Thank God I have a WoW leveling guide installed and it always pointed me to the right area for leveling.
Since many players are returning to Alliance now, and others are starting fresh, I want to point out a few Alliance leveling areas that you must put in your leveling path. That's because leveling in those areas is easier, there are more quests and you will be able to keep up a high XP rate.
1. Elwynn Forest. This might seem obvious, but this zone has to be on the list. No matter which Alliance class you're starting or which race, Elwynn Forest is the closest zone to Stormwind City, the main Alliance capital, where you can find any NPC or trainer, and there's also an Auction House. Plus, the zone is really accessible, there are plenty of quests, and even without any Heirlooms you will ding level 10 in no time.
2. Eastern Plaguelands. Back in the day, before the release of Cataclysm, this zone wasn't very popular due to its harsh accessibility and the strong creatures. The level of this Alliance leveling area, or should I say, contested WoW territory, was brought down by 10 levels or so, there are many flight masters here which makes it pretty much accessible now, and each flight master is surrounded by a number of NPC quest givers - pretty nice quest hubs if you ask me. So
3. Nagrand. I believe every player, Alliance or Horde, loves this "warm" area. There are over 150 quests here, and with a few Heirlooms, you can level up in this area from 65 to 68, and then jump directly to Netherstorm, and ding 70 there. Nagrand is pretty much an open land, offering plenty of visibility especially if you have a flying mount already.
4. Borean Tundra. You can also start the Northrend content with Howling Fjord, but Borean Tundra is much more accessible, you will move faster throughout this zone and reach level 73 much easier and quicker. Plus, if you're a skinner, you will find tons of beasts here to increase your skinning skill.
5. Mt. Hyjal. It's much better to start the Cataclysm content leveling in Hyjal, than in Vashj'ir. In the sunken world of Vashj'ir you won't start with a mount, and you will have to work your way up to the quest that grants you a Sea Horse. Plus, completing all the Hyjal quests you will become revered with Guardians of Hyjal and get access to the caster helm enchantment.