The trouble is that most later model viewfinders are usually equipped with screens that have patterns optimized for f/2.8 maximum aperture lenses. Put a slower lens on, and the viewfinder gets very dark and focusing aids can black out. (This is why special focusing screens with different patterns are made for slower lenses.) Put a faster lens on, and you still only see the D of F at f/2.8. This means that any time you shoot your camera more wide open than f/2.8, you get less D of F on the picture than you see in the viewfinder. There are screen optimized for fast lenses, but TMK they still show the D of F at only 1.8; this means that if you have an f/1.4 or f/1.2 lens, you still do nt see the wide open D of F on the screen.
Kind of hampers the effectiveness of fast glass. I wish there was a screen that would show the D of F at f/1.2. But I guess the camera makers think that most people don't even bother to check critical focus any more, and that they rely on AF and/or focus confirmation lamps.
However, IMHO, this isn't really an issue of things being terribly better in the good old days. Most of my old cameras (FTb, F-1, Nikon F, Pentax Spotmatic) seem to have come from the factory with focusing screens that show D of F at only f/2.0. Lock D of F preview, set the lens to wide open, and stop down click by click with these cameras; you will not see D of F or viewfinder brightness change until you click past f/2.0. With my 10D or EOS 3, following the same routine described above, I don't see any changes till I move past f/2.8.