I don't get the feeling that this design saw much user testing. As you describe, the domain plays an important part in helping mentally filter results and thus doesn't really merit relegation to rollover visibility.<p>What I think <i>would</i> qualify for that is the rest of the URL path.<p>Overall, as nice as it all looks, there are some serious discoverability issues. "Show options" doesn't need to be shown in as much detail as Google has it, but the plus-sign itself isn't enough to go on. The rollover visibility for the other Google products just makes it hard to find them; the header isn't that noisy with them visible. The single gear for settings/sign in is a little better than the lone plus sign, but still, such condensation is extreme, especially for the importance Google places on getting users to use accounts.