I agree with Greken, tasks shouldnt have monsters grouped together with orcs dwarves etc. Also they shouldnt be all active at all times, its better if you have to choose otherwise its too easy and its just a "passive exp boost", rewards should be hidden and interesting(Items too), not only XP, also do not use a static formula to calculate the reward. for example some rather uncommon monsters with lower spawn amounts should have a bit higher reward to encourage hunting some unusual creatures.
Also the some tasks should be short, some long grinding ones, for example warlocks shorter because they're already the best thing to hunt Exp wise.
As for the Task bosses while i think its interesting i also think its too strange and would rather see a normal task system we see on other oldschool servers that we're able to take from a NPC in several cities and only one at a time.