um, the souls are interesting additions to the classes, it depends on if they fit your playstyle.
Personally I love harbinger, tactician and tempest, but not so sold on defiler, but thats just me.
fastest way to level is onslaughts if you can get in a decent group to farm them. And then zone invasions. If not, quest. IAs are pretty low xp, and whilst the instances are great, they can take a while and have a high failure rate (they are pretty tough in places). PVP is great if you win, and its worth doing the pvp dailies.
What should you know?....you port to the new continents from Iron Pine Peak, just west of chancel. Go to cape jule first (the 2 NPCs at the portal take you to the different contients. head SE until you hit Tulan, talk to teh captain on the docks to hitch a lift to Tempest Bay, the new main city and set your soul recall there. then go back and do whatever zone you want.
Other than that, up to you really, so many options.
Instant Adventures are very good for leveling from 1-50, after that they lose effectiveness quickly.
PvP is very, very quick from 1-50 and pretty decent from 50-60, you should at least do your daily random at that point.
Dungeons are usually good experience if you are at the lower level limit for joining them, but once you are the same level as the mobs they aren't very effective, you should still try to do your daily random though.
Questing is decent at all times, but is probably the most effective staple for 50-60, before that other things are often faster.
Onslaughts can be great experience from 50-60 if you are good at them, I'm not so I don't know exactly how good they are.
The fastest method from 50-60 is chain running Hunt Rifts with a group but gets monotonous quickly if done in long play sessions.