I've imagined one of their primary internal use scenarios to be Googlers with "giant" monitors who are managing multiple simultaneous email and chat conversations. The new interface turns Gmail into something of a "windowing environment" wherein all that multitasking can be managed within the smaller, quasi-pop-up "windows".

I don't particularly like the UI changes, myself. But then, I'm not... well, to cut it short, I'm increasingly aware that I am not / am no longer Google's target demographic.