This is a simple guide designed to tell you where optimal places are for leveling. Leveling in World of Warcraft is a lot simpler than what veteran MMO players know as “grinding”. Grinding is where you find NPCs that are optimal for killing, they have low hit points, low armor, no trick up their sleeves, but they give good experience. You would then just kill these NPCs until you went up in level. World of Warcraft has put a huge emphasis on questing, which is going through the game and seeing stories unfold. The best way to level is to get all the quests from an area of your level, and then go out and do them. Make the targets of your quest your killing. This way you explore the zone better and get a bonus for killing said NPC. By no means do you have to complete every quest in a given zone, but you should watch for quests that give equipment that is beneficial.
Horde Leveling
Horde leveling is going to differ from leveling as a Draenei for the first couple of brackets.
Level 1-10
Your first 10 levels should be doing the beginning quests. If you complete all the quests in the starting area, it should put you around level 5, and you should then move to the next town over. For Trolls and Orcs, this should be both Sen’ Jin village and Razor hill.
Level 11-15
You should continue with quests in your second area and move on to your major city. You major city will also be filled with lower level quests. These quests will help direct you around your area to help you learn it. Some of the quests have very interesting story lines. I recommend that you always read the text and wait around for a few seconds after doing turn ins. This will result in some funny things happening on occasion. You should move to The Barrens and start questing in your teens. The Barrens is located east of Mulgore and west of Durotar. If you can get a group together, your first dungeon experience can happen right inside of Orgrimmar. Ragefire Chasm is recommended for a level 13-16 group.

Level 16-20
You will have lots of quests still in The Barrens still even at this level. You can expand your horizons though, and take the zeppelin located just outside of Orgrimmar and go over to the undead starting zone of Tirisfal Glades. Then head south into Silverpine forest where it will be more level appropriate. If you want to do another dungeon with a group, I would suggest Wailing Caverns located in The Barrens. In your upper teens you can move to Stonetalon Mountains. Just before level 20 you can go into Ashenvale and explore the woods there. Ashenvale will probably be your first experience with PvP if you are on a PvP server, or were not at The Crossroads when alliance attacked.
Level 21-25
At this point, you should be finishing up in The Barrens. If you want a calmer leveling experience, do the questing in Stonetalon Mountains more. If you are liking the excitement of Ashenvale, it has some very good quest rewards. If you are feeling ready to do another dungeon, I would suggest Blackfathom Deep. Blackfathom Deep is located off the west shore of Ashenvale. With a solid group of level 25s, you might be able to take the final bosses, but it will be a matter of working together well. Remember when doing dungeons to get the quests for the dungeon before you go through them. It is bonus experience for doing the dungeon.
Level 26-30

This is where a lot of PvP happens on PvP servers. If you want to participate in PvP battles and have lots of Alliance running around, I recommend leveling in Hillsbrad. Hillsbrad is located just south of silverpine forest. This zone is notorious for the PvP battles that take place between Tarren Mill, the Horde town, and Southshore, the Alliance town. If you are looking for a quieter leveling experience, I recommend 1,000 Needles. 1K Needles is located south of The Barrens. 1K Needles is normally a quiet zone with few Alliance interruptions. If you are looking for a dungeon of this level range, you are going to want to go to Razorfen Kraul, commonly known as RFK. This dungeon is located in the southern part of The Barrens, by the great lift into 1k Needles.

Level 31-35
The best zone to continue with PvP fighting and the ability to level while doing it has to be Stranglethorn Vale. This zone has the neutral town of Booty Bay on the southern tip. This zone is filled with quests to do and can be reached through a zeppelin, or a boat from Ratchet. If you want a quieter more solo adventuring time, head to Desolace. This zone is some quiet experience. The hardest part is getting to the horde village of Shadowprey. There is no new dungeon for this level range.
Level 36-40
At this level you can continue with the same zone as the 31-35 range. Stranglethorn Vale has lots of quests available to level with. A lot of people finally go to Stranglethorn Vale at this level, versus at the lower levels, because they can better defend themselves. A new zone to maybe check out is Dustwallow March. This zone is located just off to the south east of The Barrens. You can also look into going to Badlands. You have to go through the dwarf zone of Loch Modan to get to it though, so it can sometimes be a rough time. Good dungeons for this level range are Razorfen Downs, and Scarlet Monastery.