But how do you encourage people for difficult tasks? It is easy to commit myself to correct a particular spelling in a lable by tomorrow. But to fix the indeterministic bug that crashes our top client's server every now and then?
You could commit to something like, "Investigate the logs for two hours" or "inspect memory dump" for an hour. From there, one can add more detailed tasks.
Nothing. There is also nothing to encourage people from not taking forever. This metric worked well on my team until I got people who game it by pulling easier tickets and then sitting on them for a week. We still use the approach though. No approach survives a bad team. I could implement draconian rules, but that would just hamper the productive people.