These are all some addons that can be very helpful and speed up your leveling
Quest Helper/Lightheaded- I prefer lightheaded because it gives an in-game wowhead attached to your quest log, with coord's in blue, and if you click on it it gives you an arrow on where to go. Quest helper is the same thing but it goes on to the next quest when you finish one and you can't control it. When lightheaded is working weird and going back to the first page when you click on the arrows you have to hit alt f4 and start a new wow.
WoWInterface Downloads : Quest/Leveling/Reputation Mods : LightHeaded - Lightheaded
WoWInterface Downloads : Quest/Leveling/Reputation Mods : QuestHelper - Questhelper
Cartographer- By far the best map addon EVAR
WoWInterface Downloads : Map Mods, coords/compasses : Cartographer
Atlasloot- DUR. Obviously just about everyone who plays wow should have it.
WoWInterface Downloads : Map Mods, coords/compasses : AtlasLoot Enhanced
TitanPanel (I didn't use it because it bothers me but its pretty good)-
WoWInterface Downloads : Titan Panel : Titan Panel
And if you plan on doing instances...
Omen- Not many low level players have it but it's always nice to have since if your leveling you plan on being 70 at one point and everyone does alteast one instance... It lets you know how much threat you are generating compared to everyone else... Don't pass the tank!
WoWInterface Downloads : Ace: Raid : Omen Threat Meter
LeaveMeAlone- It helps for those long leveling grinds when you dont want to be distracted by anyone, it blocks whispers, party invites and guild invites by your choice.
WoWInterface Downloads : Miscellaneous : LeaveMeAlone
O.B.I. - On Button Information- It helps you by telling you how long your debuffs on a target will last or when your cooldown will pop off.
WoWInterface Downloads : Action Bar Mods : O.B.I. - On Button Information
Thats about it. Thanks for reading and happy leveling!