simple, useful, and original. It’s easy to come up with a task which is useful and original - just combine existing ideas in a new way, perhaps with some minor twists. But finding something that’s also simple is hard.
Interesting
a single task that can’t be reduced or explained in terms of existing tasks.
Simple things are often primitive (non-compound) in this way, but I doubt it's a requirement.
One user who stops using a network decreases the value for everyone else, which makes other users more likely to stop, etc.