Part of what I found insulting when I went through it: I could have been interviewing for kitchen staff, for all the information I was given. I was being screened intensely on relatively arbitrary criteria by relatively unpleasant people (as far as I could tell!), but I could get no assurance from anyone that I was interviewing for a position that I would find interesting, or even that I would be working with people whom I liked.

When you interview with Google, you're operating on faith that the interminable, silly process will result in a position that makes you happy. But there's rarely any assurance that you'll get a job that you enjoy, or even that you'll work with people that you don't find extremely annoying. This sort of interview process works better for inexperienced people than it does for more senior employees, who tend to want luxuries like career growth and cultural fit.

I wasn't insulted by the CS questions. In fact, they were the most challenging and nontrivial questions I'd had in any interview (one of the first round engineering* ones included writing up an inductive proof of my solution and emailing it in later).

* I applied for both APM and dev, and chose the APM route after passing first round for both.

What I found insulting was the lack of communication, complete disregard for my time, and general attitude of the HR people. It shouldn't take three weeks to let me know if I passed an interview, for an internship, when I tell you I'm under time pressure. I had an exploding offer from Microsoft, which they graciously extended so I could keep interviewing with their rival, and Google still took their sweet time. I was told I couldn't write the essay until they'd decided I'd passed the second round interview, even though it would have cost them nothing to let me write it and then throw it out if they decided I hadn't passed the interview. And when I turned it in, I was told it had to be "reviewed by the hiring committee, which meets every Friday, but they're not going to look at it this Friday, you'll have to wait a week and a half." When I basically told them that I wasn't going to let my other offer expire, they begrudgingly looked at it that week, but they made it very clear they weren't happy about doing me the "favor".

As someone else said, Google gives the impression of being full of huge egos who think that everyone is just dying to work for them, and that working anywhere else is a distant second choice. It's arrogant and egotistical. Think that if you want (even though it's not true, at least among Stanford CS students), but it shouldn't come off so blatantly to potential recruits.

Edit: timr just reminded me of something else - the interviewers seem trained to emanate disinterest. They were the least friendly interviewers I've ever had. And like I said, Google seemed at least a bit interested in me, since I got through both rounds of interviews, so I doubt it was personal.