Try using Disk Inventory X to see what is taking up space. It is very specific and it will give you a visual representation of your hard drive. Plus it is open-source/free.
Disk Inventory X
Here is a screenshot of it on my mac.
I know you said you already reviewed all visible files, but this also shows you your library and where all of your applications store data (~/Library). I think Lion hid this directory, which would explain why you didn't see it. If you want to access it, go to finder ->go -> go to folder and type in "~/Library". I used a command to make it permanently visible, but I do not remember it off the top of my head (I did it when Lion was released.)
Merry Christmas!