The tasks, whether they are OoW tasks or DoN tasks are not soloable by everyone. DoN has a fixed set of tasks, and if you are too low (or not a good solo class) you won't be able to solo them. The OoW tasks change as you level up. However, at higher levels they become increasingly unsoloable.
For example, at 49, my chanter was given the task to kill 10 scalebones in Burnt Woods. That one was a great task. It was difficult, but not impossible. Once I dinged 50, I went and got another task. It told me to kill 10 wyverns in Skyfire. Well, it's not easy to solo wyverns as a chanter. That one was impossible to do. Even my druid (who solos well I might add) had trouble with some of the higher level ones, and couldn't solo the DoN tasks at 62 or 63.