This is basically just a short guide to show people where to go to get experience and at what levels.
For each of the 6 starting zones, players should spend between level 1 and 10-13 there.
Darkshore: Recommended levels: 12-20/21
Main second Night Elf zone, this is a very long, thin zone that van be a pain to get around in. It also has a dreary atmosphere and some relatively hard quests. Located Southeast of Teldrassil, east of Felwood and north of Ashenvale.
Comments: Personally I hate this place, as it is very big, has a dreary atmosphere and it is hard to do the quests there. 2/5
Silverpine Forest: Recommended levels: 12-20, with a few quests for 20+, and a dungeon for around 25-30's.
Main Second Undead zone. There are quite a few nice and easy quests here, so it is one of the better of the second zones, however almost all of the enemies are undead, and the area is quite dreary. Located South of Tirisfal and North-East of Hillsbrad Foothils
Comments: One of the better second zones, with lots of fairly easy and fun quests, but a little lacking in atmosphere. 3.5/5
Loch Modan: Recommended levels: 11-20
Main second Dwarf and Gnome zone. A nice environment with quite a few quests, and an area of level 18-20 elites in one corner. There are also quite a lot of the lower end of the level range mobs here, and not may above 15, making it harder for the higher end of the teens to level up. Located East of Dun Morogh, south of Wetlands and north of Badlands.
Comments: Good for early teens leveling, but gets much harder for the late teens, but with a nice atmosphere. 3/5
Westfall: Recommended levels: 12-18
Main second human area. Nice and sunny environment, with lots of travelling quests. The mobs here are mainly in the low teens. This area is also strongly connected to Redridge Mountains, which forms the later half of the human teens. Located south of Elwynn Forest and east of Duskwood.
Comments: Nice atmoshpere but only for the lower teens, with a lot of delivery and movement quests. 3/5
Barrens: Recommended levels: 12-25/30
Main Orc, Troll and Tauren Second Area. The Barrens is a monster of a zone, containing the largest level span of any zone. While there are a huge amount of quests, the environment is very repetative and gets quite boring fast. Situated in between Mulgore and Durotar, connected to Ashenvale, Dustwallow, Stonetalon and Thousand Needles.
Comments: Very, very big with a huge level range and lots of quests, however very repetitive quests and scenery make this zone become boring... fast. 2/5.
That is just the second zones, i'll post some more after getting some feedback, and compiling them of course.
Edit: continuing on to the 20-30 zones:
Ashenvale: Recommended levels: 22-32
Ashenvale is situated south of Darkshore and is mainly the third Night Elf zone. This zone is suitable for both horde and alliance, as there are towns for both, but most of the quest are alliance, and the alliance town is bigger and better situated. Most of the mobs here are level 22+, so it is not suitable until then really. Ashenvale is populated by all kinds of monsters, from Murlocs and wolves to Satyr and large walking tree trunks . The scenery in Ashenvale is nice, with cheery music and bright areas.
Comments: A Large zone with one big alliance town in the center and two small horde towns on either side. Some nice atmosphere to, but the quests are very hard. 3.5/5
Stonetalon Mountains: 16-25, with some parts suitable for up to 28's
Stonetalon is Northwest of the Barrens and South/ Southeast of Ashenvale. The mobs in it are between 16-28, and one half has mainly humanoids, and the other mainly beasts/elementals. It is mainly very dusty and deserty, except for the peak which is all green and pleasant. This is also where the Alliance town is situated. The Horde town(s) are situated further south, near the barrens entrance and half way up.
Comments: A small and simple zone with a variety of monsters, but with few quests, for the alliance at least. The atmosphere is similar to the barrens, but with a grassy area to the north and a charred area to the west. 2.5/5