Some things I've noticed, may not be correct, but after 10 hours of play I'm at level 20 which isn't too bad. This is mainly for speed, I'm all for leisurely leveling, but efficiency never hurts:
While the other paths maybe more fun, Settler helps you level the fastest. Having your own buffs is immensely beneficial. Always make sure you have enough supplies to completely power a hub, just incase. If there's a vending machine, craft it, they sometimes sell blue weapons and armor for cheap.
Once you are past the initial crafting introduction missions drop all the quests that actually have you create something. Likewise either equip the item or vendor it, don't salvage.
If you want to save up for a mount pre level 15 that's fine, or you can just vendor everything after Thayd and you'll have enough by the third hub.
Always make sure you have a run speed boost from a settler's node if at all possible. Even if you have to go slightly out of your way to get it.
For Stalkers(and maybe others)Level as a tank, but use dps weapons, any ability that isn't aoe toss out. That means drop impale as soon as possible. Time spent stealthed is time spent slowed and there's no need to line up shots when you can just aoe. Pull absolutely everything and burn it down. Put your amp points into lifesteal, brutality, and cd reduction. Tanks can easily take prime/superior mobs and 2-3 person quests solo. The key to speedy leveling is knowing where to go for your quests and the most efficient way to complete those quests and to not wait for anything.
This goes with the above, but if there's a prime or superior mob in the area it probably has a quest tied to it that you will only get if you kill it.
Use pounce to travel faster, with max movement speed buff and pounce usage on a road you travel very quickly. Speaking of which I think +50% movement out of combat buff and pounce usage might be quicker than a mount, I don't know the mount speed, but it seems slow.
If you complete a challenge, cool, if not, do not repeat them.
It all seems rather obvious, and there's a few more tips, but I can't remember the ability names so I'll post them later. Hopefully, it might be helpful for someone.
