I actually came out of my Google interview thinking I'd blown it. I was making contingency plans in my head on the plane ride back, of what business I'd start next, since I apparently wouldn't get the job. Then the recruiter woke me up the next morning (I had a red-eye back) and told me the interview feedback looked great and they were fast-tracking me through the hiring committee.
The lack of feedback does suck, but I wonder if it says more about our own ability to judge our competence than about the interview process itself. As btilly points out, companies in the U.S. are almost required to not say what you did wrong, for fear of lawsuits. Plus, the hire/no-hire decision usually comes down to a visceral reaction, so the decision-makers may not even know why they decided one way or another.
Also - if you think the lack of feedback for interviews sucks, it's ten times worse when you're doing your own thing. I was an entrepreneur before Google, and we launched like 4 websites before I threw in the towel and got a job. Each one of them was met with a resounding...silence. Nobody saying we were bad, but nobody saying we were good either. We were just irrelevant. That's the way most startups go: if you're successful people will just quietly use your product and never acknowledge you, and if you're unsuccessful, people will just quietly not use your product and never acknowledge you. The only startups that get masses of haters are the ones where the founders were previously famous but royally fucked up (eg. Cuil), and the only ones that get massive acclaim are the ones that solve a hair-on-fire problem in a particularly spectacular way (eg. Google).