It makes a new folder called "foldername-grey" if one doesn't already
exist.

It doesn't go in to subfolders.

It ignores files it can't process.

It will use all the cores on your computer.

It might be slow on a Mac. I'm not sure about this part.

It should run on just about anything - you can give it to your
Windows-using friends.

It will use the original filename unless a file with that name is
already in foldername-grey, in which case it will append a number. It
will never overwrite an existing file.

It's really simple - just click the button and select a folder.

It doesn't support RAW images (but it does support jpeg, gif, png, bmp
and more).

It doesn't keep metadata like jpeg's EXIF in the converted images. The
biggest problem with this is that pictures that are taken sideways and
automagically flipped upright by some programs won't be in the converted
images.