MAC addresses?
I'd like to see them include an option for the MAC addresses. I would actually trust that more than cookies, which could be copied or counterfeited.
However, I still feel like the #1 problem with ALL email is spam, and the #1 feature I want is a better tool to make war on the spammers. Something like SpamCop on steroids to track down the various forms of involvement, with the potential power of Google threatening the supporters of spam. I'd be glad to donate a bit of my time and my human intelligence, such as it is, to help nail the spammers. After all, we spam-haters vastly outnumber the fools who feed the spamming animals, and the spammers can't hide from us without hiding from their own fools.
On the other hand, the prevalence of nasty phishing spam in Gmail is additional evidence of how evil Google is becoming. Lots of the spam is abusing the reputations of legitimate companies. I'm just waiting to see a real Disney ad running next to the fake Disney movies scam targeted at children... That would be a screen shot worth a thousand words about the evil of spam.
(I though the Register had an icon for spam? Anyway, the hand grenade scatters fewer fragments than the spammers do.)