I first learned of him from a post on Slashdot about a cool remote controlled door hack in a college dorm. His roommate posted and basically said, "this guy is brilliant, it's just the tip of the iceberg - he posts what he's working on at bradfitz.com, check it out." This was in...1998? 1999?
Seeing him start to post a daily journal, get frustrated, then write LiveJournal was fascinating. Posts on the problems he faced scaling it, open discussions on caching architecture (leading to a LiveJournal user suggesting he investigate slab allocation technique that led to memcached), identity (leading to OpenID), moderation, etc. were always illuminating. Moving to Go because he understood what problems needed solving just made sense. Rewriting Google's downloader because it sucked? Well, of course he would.
I don't know Brad other than occasional comments/responses from the early livejournal days (user number ~10K, after procrastinating actually joining for too long), but it's clear that he draws inspiration from overcoming roadblocks.
Taken in that context, leaving Google is completely understandable - having a trillion dollar organization there to remove roadblocks would subvert a key driver for his personal innovation and inspiration.
Good for him that he recognized this and is taking the leap into the unknown. Can't wait to see what he comes up with next, but I'm guessing the real treasures will be found in the process and not the final outcome.