The perception is a major upgrade but in reality, it was a minor (maybe medium). By that time, MS had righted the Vista ship, but it was too little too late, in the eyes of the consumer. Vista was a dog in the eyes of the public (and enterprises) and they chose to wait until win7
I've used both vista and win7 and its definitely different, the look, feel even to a degree the performance.
Between OSX 10.5 and 10.6 you really can't say that, visually speaking you really couldn't tell which OS you were using. There was no consumer facing major feature that you could point to in SL.
The iMacs at my school have Leopard, not SL, and I didn't notice the difference until some 3rd party software that works on my SL MBP at home didn't work on those iMacs.