Writing a thesis acknowledgement: should I address the person as "you" or use 3rd person?
I'd go (and did go) with the 3rd person format, for a simple reason: It's not a letter to that one person, nor a (handwritten) dedication/signed copy of a thesis. Instead it is something that is printed and which is (hopefully) read by more than this one person.

So you talk to a broad audience about that person, hence 3rd person. Only if it were a handwritten, personal message in one particular copy, I would ever go with "you".

(Another exception could be a dedication. In contrast to the acknowledgements the dedication is more specific and -- usually -- to one person or specific group of people, like family. In this case 'you' might also make sense, because here you address them personally.)