Where are the disruptive start-ups in Web 2.0?

Umair Haque thinks Web 2.0 economics is causing start-ups to focus more on getting acquired than creating real innovation. He wrote:

"What made the Valley cool was it's refusal to think small, and do truly disruptive things. But getting a small change acquisition to essentially extend a Yahoo/Google/etc product line sets incentives for incremental, not disruptive, innovations and models."

The observation is a good one, because of all the current crop of start-ups I can't think of many that have the potential to become the next Google or Yahoo!. Some start-ups are doing technically innovative things - for example Ning's goal is to enable non-developers to create web applications and Flock is attempting to build the next-generation web browser. Both are technically ambitious, however I can't see Ning becoming the next Sun.com or Flock becoming the next Netscape.  

I also don't see many start-ups attempting disruptive things in the non-geek space. I see plenty of events web apps, tons of RSS Aggregators, lots of AJAX-powered office apps. But what about Web 2.0 applications that will tackle things like health, finance, education, government? Healthline is the sort of thing I want to see more of. It's a new medical search engine that promises "The Web's Best Health Information at Your Fingertips". Tony Gentile, HealthLine’s VP Product Management, has more information about Healthline on his blog.

If you know of a big-thinking Web 2.0 application or service that is truly disruptive, please share it in the comments. I'd like to explore this much more.