Like said before blade dancer is the easiest in the game. Warlock is arguably the hardest in the game. You lvl all the way to 20, 6850 xp in total. Quests are not the highlight of the game, interaction with opposing factions is, however you make take to that. This can be done in many ways:
1. Arena: Getting a certain number of allied players against another number, usually the same amount, and having a fight to the death. Your oppenents can be of the same alliance, but in arena they are a temporary enemy. There is 2v2, 3v3, and 5v5 with lvl ranges of 4-6,7-10,11-14,15-18, 19-20. These are not set in stone however, if all of your team is of the highest lvl in that division (6,10,14,18,20) you can face enemies in the next division if one is the lowest in his division (7,11,15,19) and his partner is the same level or one above (8,12,16,20). This may seem confusing I realize. Arena is popular and is mainly controlled by high amped rangers so if you want to be strong in arena make a ranger and amp it high. You can use certain things to boos abilities but only in the 10 seconds before the match starts.
2. PVP: Player versus Player, this is where you go 1 on 1 with a player of the opposing alliance. You walk up to the red name player, do some type of skill that does not dmg the player or hinder his abilites (Warlocks cant do this, they get an small advantage) and if the red name signals you kill each other. Health/mana potions are counted as cheating and others may kill you for it.
3. Ganking: The most infamous thing in the game, this is where you kill an opposing player without any signal. Theres no rules really, its just kill everyone. If you are a ganker people will kill you on sight, so if you want to be good at this, you will have alot of haters and will get ganked in return. Ganking can also be a way to show disrespect or even hatred toward another, if someone annoys you, ganking him is a common method for revenge. Lots do gank, I don't personally and don't usually hold grudges on the ones that gank me because I can usually get them later. Either way you will have to be strong or you will fail miserabley and still be hated.
4. Wars: This happens in 1 way generally, one side is pissed at the other for ganking and sends a large group at the other's camps to kill as many as possible. Great fun, usually last about 10-30 minutes. Easy to start too, especially if you a popular player with alot of influence.
5. Other: Don't be supprised to see a player of the opposing alliance walking around you arena, just kill him and get on with whatever your doing.
These are the many ways you interact with the opposing alliance, if I forgot anything tell me and to the author of this post Good Luck out there :)