The challenge is to come up with a way of marking out the importance of a bug, without having to spend time thinking about important levels. A simple one to five or one to ten system is quite vague, and one person's 4 is another person's 2. So numbers aren't ideal.
I've trialled a simpler system: Critical, Need and Want. That works pretty well, although the Need category is quite overpopulated. It's clear three levels isn't really enough once you get past 30 or so active bugs - of my current list, around 25 are "Need", with 2 "Critical" and 3 "Want".
The second issue is that, while a client might think a particular bug is critical because it occurs in their favourite part of a site, I might think of it as relatively minor as it's not affecting sales.
Other bug systems have different levels of success with this, and among them Drupal has probably the best system: Critical, Major, Normal and Minor. That's close. But it's not quite there.
I want a system where each level ties to a concept of importance. With moderately clear delineation between levels. Where I could let a client could see the prioritisation of a bug without worrying it would upset them.
Here are the contenders I'm looking at:
Not bad, but people disagree about whether "Need" or "Important" (and even "Critical") are highest importance. Not quite intuitive enough.
Clear delineation, but only works for bugs, not other issues, feature requests and additions.
I dislike "urgent" as it conveys an importance of time, not priority. And a client will always think everyhing is "must fix", but on the plus side the delieation is clear.
Please feel free to add your thoughts or your own system in the comments.