I'm guessing people here are familiar with the idea of 80% of the results coming from 20% of the effort. Although the numbers don't hold in every situation, the basic idea works for most. I'm guessing people are also familiar with the Lean Startup, which emphasises quick 'n' dirty versions to enable rapid testing. People might not be familiar with Boyd's OODA loop (observe, orient, decide, act) -- but the idea is you want to move through the loop as fast as possible, and gain a strategic edge on the opposition.

I'm guessing people are also familiar with Apple, which follows a different philosophy -- aim for 100% every time, even if it means your products cost twice as much as the competition.

These philosophies aren't really opposed. Sebastian had a good post about this (which I can't find right now). You want to aim for 80/20 crappiness, or 100/100 awesomeness. You don't want to get stuck at the sucky zone in the middle.

I agree.

However, how can you decide which things to 80/20, and which things to 100/100?

I've realised recently that I might be overly biased towards the 80/20 approach. How can you tell when you should be aiming for greatness, not quick completion?

I think when you've been working on a project for longer than a week, it's time to reconsider. It's generally not fun working on something you want to finish as soon as possible. Hopefully you finish it quickly and it's just another chore. But walking the quick 'n' dirty road for weeks, with no end in sight, is pure misery. 

I've had this experience recently, with a freelance project I expected to take 4 days, and has so far lasted 2 weeks. Slogging at it day after day was depressing. As soon as I realised that it was going to take longer than I thought, and that I should really do a good as job as possible, the project became much more enjoyable.

A much larger example is American intervention in Iraq, Afghanistan, and other places. They were hoping for a short, quick campaign. As things got bogged down, they kept talking about pulling out as quickly as possible. Things just got messier. If they'd planned from the start to be there for the long haul, things might have got resolved sooner. (I'm aware that this may be more the fault of election-wary politicians than the military strategists). Then Obama is thinking of invading Libya, "just a lightning operation". Yeah...

Summary: if something's worth doing, and it can't be done quickly, then it should be done well. Strive for greatness and mastery in everything you're committed too. That way lies happiness.