[bonch]

I don't see what's changed in 10.7 regarding windows. Can you point the app that had multiple windows, but moved to something else?

Other than the APIs for fullscreen support, there's little that can be said without violating NDA. Other Apple applications have gone single-window over the years; Logic merged its windows two major versions ago, and Final Cut Pro X just did the same.

Steve Jobs has always preferred single windows. Before OS X 1.0, the toolbar button was instead a toggle for single window mode.