I dont quite understand why these really obscure tags get rewarded by google. As a web developer, I havent even heard of some of these tags. if they are almost never used by most websites, why would google reward people that do?
This is a fascinating thread. Thanks to the OP!!
Darn it...

Now I have to explain some stuff and people are going to get mad and argue with me.
This is all UNPROVEN to the community at large... Parts of it are PROVEN to me, but 2/3 others who have tried to reproduce my results have failed to do so for whatever reasons. I only know the details of my own work so that is all I can speak to.
Statistical learning explains it pretty well, but its just a guess. Imagine you are google and you have a database of 2 million web spam sites that have been reported to google by webmasters who were policing their keywords. Wouldn't you want to run some stats on the spam sites and see how they differ from top 10 results in general for their keywords they target? Ultimately you end up with something essentially representing a relative likelihood of a tag to be used in a spammy way. The leaders would be deprecated and obscure tags that nobody uses. Thats why I say it doesn't matter which tag is #1 I just matters that if you have a spammy site and want to try to beat the auto-classification then you might consider higher ranking, but still appropriate usage tags. Prior to Caffeine/Mayday this tuning trick made my clients about $3M US... It doesn't appear to matter as much anymore since Google's algorithm is soo different and only has 4 significant factors
1. domain match
2. dmoz & yahoo links
3. keyword density (if you don't have 50+ keyword matches per page (full source... check it out... even HTML comments help now) ... )
4. page rank (Its back in a big way... It looks shaky because it is a secondary sort but it pumps traffic from google... A new twist on it is that if you want to maintain High PR you need to link to page... PR has been tweaked to favor home pages and index pages, another totally exploitable pattern)
and I'd say the unofficial factors that help those 4 work efficiently are
A. site structure
home pages need to directly link to pages of importance, anything more than 1 click removed get 0 link juice... that's why long tail vanished for online retailers... home pages linked to category pages linked to product pages... product pages fell out of the serps... retailer loses %25 revenue from Google... sound familiar? Took me a month of research to prove it to myself with my logs and analytics.)
B. web site age
Not domain age.... Google uses "Date First Indexed" not whois... why would they rely on an externally controlled factor when they have a better and more appropriate analog in their own system. There is a reason they don't share "Date First Indexed" ever... WayBack Machine is better tool than whois for website age and will probably save you and others from being scammed for worthless old domains that have never been websites.
C. fresh content
Newness counts a lot now.
D. Media file links and Microformats
Those star reviews in the SERPs come from your content!
E. Legitimacy Pages
Contact Page, About Us page, Press Page, Privacy Policy, Security Page, Management Page, Robots.txt, favicon, apple-touch-icon, Phone number, physical address, etc...
F. Fiduciary Duty Factors
AdWords Spends & Google Checkout & partnership contracts ... they have a legal requirement to their shareholders to do this, and IF they don't their requirement is to continually revisit this notion in other ways. Obviously, they are in the business of making money. Content isn't king. Money is.
The unofficial factors don't get you to the top... they help you stay there and be competitive.