So here's a useful mnemonic to help prioritise tasks or projects. Think about the RETURN:
R: Resource
E: Effort
T: Time
U: Urgency
N: Necessity
(OK, there should really be another 'R' in there but I'll let that slide. Perhaps it stands for 'Argh, I've forgotten one of the words...')
Only tasks with high urgency and/or necessity (UN) should justify a high level of resource, effort and time (RET). And urgency versus necessity (AKA importance) is a separate question.
Anyway, I was introduced to this mnemonic at London MeasureCamp 13, in a session run by Andrew Gerhardt - but I don't know its origin. It sounds like it has perhaps come from a book on project management. I'd be happy to credit if anyone knows.