They're also colour coding the number of people - though they've gotten a little confused with it. They're using green for those with hints, and greying out those with private trees - however one of my cousins with a private tree isn't greyed out because it's green... but it also doesn't show the little padlock either.
I have found the "unlinked trees" useful in the past - but as Peter says, only if they related to the person who did the test!