While I understand that they have legal issues giving out feedback, but I don't see why they can't dish out simple numeric scores for each of the interviews. That will at least make the candidate aware of how much he needs to improve, or if he can even improve enough to apply again, or it's just not worth retrying. This is specially expected from Google since they ask you to prepare a ton of material for around a month which is real hard work.
Anyways on to the real stuff. I cleared the only phone interview, which was very basic, shared Google Doc coding for about an hour. I was then called for a set of 5 onsite interviews, which were 45 minutes each. While I won't give out the exact questions because I want to abide by the confidentiality agreement, I will just try to summarize.
Two were design and implementation, two were algorithms, and one was algorithms and implementation, all of them required writing on the whiteboard. I think I did well on 3, ok on 1.5 and bad on 0.5. The average level of questions on a scale from 1-5 was a 4, 5 being the toughest, and I thought me doing good/ok in 4.5 was enough for an offer. Although I should make it clear that I did use hints from the interviewers sometimes, but not always.
If you ask me why I think I wasn't given an offer, I would say it must have been a combination of me not being from an ivy league university, although I am masters in computer science from a within top 50 state university in US, with a near 4.0 GPA and I have around 5 years of work experience. Or it could have been that I might have not paid attention to little details which I thought were not important, but they really were, like forgetting an obvious parameter while designing a method of a class which I quickly rectified once pointed out by the interviewer. Or it could have been that in all my solutions I started out with the most basic worst time complexity algorithm (because I didn't want to stare blankly at the whiteboard for half an hour thinking about an optimal solution), and over time improved it, sometimes with hints and sometimes without. But I have to say I was really proud of at-least two non-trivial solutions I came up with. This sort of uncertainty is exactly the thing that could have been avoided by a simple feedback email from Google instead of them just informing you on phone that you weren't good enough.
I hope your Google Interview experience wasn't like mine, but I would really like to know how it went for you?
Thanks.