Reviewing the query letters has been helpful for me - yes, it's all about my needs, not yours - as it's forced me to examine how I make decisions when I'm reading letters. And the main points I've gleaned are these:
1. If I like the sound of the story, I'm prepared to overlook missing information in the query letter - therefore, the story needs to be described well.
2. If I like the sound of the author, I'm prepares to overlook a query letter that is otherwise lacking. Liking the author doesn't mean liking their biographical information - it means liking their tone. A lot of query letters read the same - with a flat tone - and that's probably because writers are taking them seriously, which is fair and reasonable. But an author who shows me a bit of personality - an 'I love' or 'I'm passionate about' or 'I came up with the idea for this novel while standing on my head' - is going to make me want to read what they've written.
3. I often work on instinct and there's some stuff I just can't empirically break down about why I like some letters and not others. The authors I've found in the slush pile have all - without exception - had fantastic letters. I got a feeling when I read the letter and then it was borne out when I read the manuscript. Wonderful writers always write wonderfully, regardless of whether it's a query letter or a novel or an email.
So I guess what I'm trying to say is: don't be too rigid in your letters. The basic structure is: describe the story, tell me why I should read it and tell me a bit about you, and write the letter as if you're writing to someone you want to start a relationship with. The agent-author relationship ideally endures for years, and we're all human - we all respond to emotional cues, even in business (perhaps especially in business) - so when someone sends a query letter that makes me laugh or makes me feel like they have a wildly beating heart, it makes me want to work with them. It makes me want to work on their manuscript to get it ready for submission. It doesn't matter so much if their novel isn't 'perfect'.
In the US query letters are often the only thing an agent will look at first up; in Australia we tend to ask for chapters as well. So the query letter in Australia probably isn't as critical, but it's still important. It's my first introduction to an author I may well end up working with - and it's always a thrill when I get that feeling - you know, that feeling - when I read a letter and suspect that it may just have been written by an author whose work I'm going to LOVE.