Del.icio.us just announced a new feature, created due to popular demand, that allows users to save links privately — this is astonishing, like Starbucks introducing a 4oz decaffeinated coffee that sells for 75 cents — it’s completely antithetical to the whole concept.

Could this be the beginning of the privacy backlash against the Web 2.0 “social” lovefest? If nothing else, it raises fundamental questions about the mainstream viability of the Web 2.0 value proposition, which assumes that everyone will find value in sharing everything in public.

Here’s a Web 2.0 test: pick any of the hundreds of Web 2.0 apps the enable users to “share” and see if you can find on the site an explanation of WHY sharing, tagging, and being social with your media is a GOOD thing. Do any of them explain the value of sharing, or do they all just assume, a priori, that all users will think this is just the best thing ever?

The privacy concerns are all around, from Google’s subpoena fight to the permanence of everything we post online, good, bad and ugly (thanks, Google).

Web 2.0 only works if we’re willing to cede any grasp on privacy by sharing everything we do online — even everything we think, through tagging, commenting, voting, etc.

What if most of us decide that the Patriot Act is bad enough? How can Web 2.0 be successful in an online world where 60% of long-time Internet users are deleting cookies because of privacy concerns?

Maybe this is the real digital generation gap. According to the Jupiter Research study where the 60% stat comes from:

Consumers showed a clear age divide when it came to paying attention to privacy or security. Only 33 percent of respondents between the ages 18 and 24 said they paid attention to stories and articles about Internet privacy and security, compared to 62 percent of those ages 45 and older.

It’s been much discussed (including here) how the 25-and-under crowd is more than willing to expose themselves on MySpace.

Maybe Web 2.0 just needs to write off everyone over 45, and everything will be fine. (Given the disorienting and cluttered design conventions, it pretty much has already.)