that's about all they have in common. の意味は？

下記の会話について質問です。


Sam
What do Britain, Greenland, Australia, Hawaii, and Cuba all have in common, Rob?
Rob
Are you planning your summer holidays, Sam? Or is it that they're all islands?

Sam
Right, Rob, they are all islands, but that's about all they have in common. There's as much variety in the world's islands as in the people who live there!
----

Or is it that they're all islands?
that's about all they have in common.
この２文の意味を教えてください。