Clean, modern, and filled with nice Canadians, YYZ is a very good representation of the Torontonian spirit. The hallways are always kept pristine clean, and the Tim Hortons is always a must-stop for me. The place has truly stood the test of time, and it feels like nothing has changed when I go back every couple of years.
It's the only airport I've been to that has automated "check-in" machines for U.S. customs, so the customs folks can just let you through without checking thousands of blue forms that are mostly identical, by hand. There are also plenty of conveyor belts so you won't be running long distances like crazy to catch flights (I'm looking at you Vancouver airport!). Along with the free wifi, and the plethora of bars playing hockey, this is a grade above most of the other airports I've been to.
Definitely worth having a layover connecting flight here!