Hi Kirk,
Something that has been getting me on the OS X version of PM4 is that the F key does not toggle the details and thumbnail panes as it does on the Windows version. In other words, on Windows in the preview mode, hitting "F" will cause the panes to disappear and reappear. On OS X, "F" causes the panes to disappear but requires hitting "R" to return to the paned view.
Personally, I'm a photoshop nut that hitting a key repeatedly will toggle or cycle through modes so I see the way the Windows version works as best.
Cheers,
Aaron