The small D-rings are too small, the large D-rings are too large, and the butterfly has a nice size D-ring.
It would be nice if D-rings didn't flop around, so they always were right where you left 'em, and so they stuck out. The small D-rings have a bend to them which helps, as does the butterfly's, but they both fold out of the way when you don't want them to.
The new saddle that John Paul was talking about(what the heck is the name of that thing?), has floppy round rings, the worst design I've ever seen, IMO, but I have not climbed in it, so I could be wrong.