The main problem is not diff but merge. You currently can't merge changes to images, textures etc. The whole advantage of DVCS like git is that it allows to work in parallel and merge effortlessly. The day we can merge photoshop, maya and 3D studio files central repository for art files will not make sense any more. But that day has not come, yet AFAIK.

What kind of artist work can you practically distribute between multiple people and merge? Most things like this are broken into pieces and each piece is tracked separately and assembled later.

I think this is a big mistake programmers make when trying to create VCS for other industries. A lot other fields with binary files don't care about merge as much. Like electronic design automation: track every part's history and even every module on a board or die layout. But, practically it's difficult to have two people productivly work on the same small piece simultaneously.

[edit] I also think a bunch of the reasons programmers require merge are because code is organized into files. Organization into files is arbitrary and often doesn't correlate particularly well with logical structure.