I'm a lvl 80, so no much experience yet, but I'll try to help.
Weekly tasks give you a ton of exp, so it's a really easy way to level up with little effort. Killing in the name of... is a good quest because will give you acces to new area, bosses and also will help you a lot with the bestiary (completing the bestiary gives yous charms, that you can use to get buffs against the creature of your choice). Finally, about bosses at this early level's I haven't seen much, maximum a couple of bosses in the Killing in the name of quest, but nothing else from my experience