I left google because I was working on v. 8 or 9 of things, and it just got boring adding new features to giant systems that weren't that interesting. I wanted to work on core software again, write a new system, or have freedom to make improvements and change. I didn't want to work on a multi-million line java program with relatively fixed architecture and slow evolution over time. There were tons of great things about google, good pay, the best people, awesome benefits, the gym, food. Really the quality of devs when I was there for almost 10 years were excellent. I only came across one asshole manager in my time there (unfortunately he was my manager, but he's left since).
The technology of test, development, source code editing was the best. But, there weren't that many fun projects.