Lapizuli - 5:18 am on Mar 4, 2011 (gmt 0)
In My Opinion: If you're looking for 'the one thing' that's the cause of the drop, it's the web page ... Don't worry too much about the source code, look at the page in the browser window ... The answer is right in front of you...
I wish I could see it.
As an example, one of the biggest hits I took was for a low-competition keyword. My page was topped by three entries from two keyword-domain-name websites that don't answer the query with any authority or completeness (in other words, they're rewritten, vague, wordy meanderings around the keyword).
Since mine was based on personal experience and also had 100 commenters offering theirs, mine does answer the query, as does the other result (call it site 3) above mine.
Admittedly, the two keyword-domain websites had floated to the top BEFORE the update and now came in stronger, showing for more keywords, and with extra results. They have a cleaner aesthetic than site 3 or mine, but far less information and absolutely no authority - in fact, the top site removed content after I'd filed a DMCA against them for plagiarism. That same website changed all its content - rewrote it all - and it only helped them move up more. They are the "low quality but not spun" kind of content that I think Google was trying to target. My page has flaws; it could be made more useful; eventually other pages will be more useful than it, but right now, it and the result above it are the best out there on the query, at least that I've ever found.
I look at all the pages in question and have a few clues, but it's nowhere near obvious. The two domains and site 3 that overtook me have a more classy aesthetic, fewer ads, more white space. That's it. That doesn't seem enough by any means. Those would be awful signals of quality ON THEIR OWN for the Wild West Internet we're in right now - like expecting the general store to have a fancy facade. Not practical if they're busy getting the job done and don't have extra funds.
The source code IS revealing some patterns, but I need to think about it some more.