So here's a useful mnemonic to help prioritise tasks or projects. Think about the RETURN:
(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.