Almost every World of Warcraft player wants to reach level 85 as fast as possible these days, to be able to start arena or raiding with their friends. Leveling is just a big, fat obstacle between level one and the end game tons of fun. That's why I've always been using an Alliance leveling addon (yup, I play only Alliance, you got me), and here are for aspects that I like about it.
1. Complete leveling guide. This means it has a full, step-by-step quest path, from level 1 to 85. Even if I level up a twink for low level battleground PvP fun, I would never resort to the basic WoW quest helper. A player made Alliance leveling addon is always ten times more explicit that the WoW quest helper, giving you tips and tricks on how to solve the difficult quests, and also tells you the exact location of all quest items, mobs and objectives.
2. Easy to install and update. The Alliance leveling addon I've been using can easily be installed, and it's easy operate just as easily as any other World of Warcraft addon. It works perfectly with the game interface, and even if you use an outdated version, you basically don't get any LUA errors. Updates are free, but you have to update your version manually. At least that's the only way it works for me.
3. Slick interface. This is actually the feature that I like the most about the leveling addon I've been working with. Doesn't occupy a big chunk of screen, and it's adjustable. You can choose to view more than one step from the guide, which helps, because sometimes having a better overview of the actions you must complete in an area will allow you to finish all the quests more quickly.
4. Automatic step tracking. The leveling path is step-by-step created as I said, and each time you complete a step in the guide's leveling path, the guide will automatically update, displaying the next objective that you should complete. Of course, in case you want to skip certain steps, you can scroll forward. Also, if you want to start another zone, all you have to do is access the quest path for that particular zone. It covers most of the zones in the game.
Now, if you want to reach the WoW level cap in the shortest time possible, I can only recommend you a player-made Alliance leveling addon. I'm saying this because using one, it took me only 3 days and 22 hours to level up my last ally character.