I would appreciate it if you would go into detail if you wouldn't mind. Your thoughts or any good links would be appreciated. Hopefully this won't hijack this thread.
Film is less sensitive to the angular dependence of the incident light rays. To put it simple, it doesn't matter whether light enters film perpendicular to it or at an angle. Even if light isn't absorbed in the `first' layer, since film has a certain thickness, it will be absorbed slightly later.
Digital sensors only have one `layer' where light is absorbed -- and the absorption is much more sensitive as to whether the light comes in perpendicular to the surface or at an angle. If it enters at an angle, the absorption probability is lower.
This leads to a higher amount of vignetting and is particularly significant for wide angle lenses.
Modern sensors try to combat this by putting micro lenses on top of the sensor so that the light beam is focussed again and hits the surface of the sensor at a `more perpendicular angle.'
Also, sensors are more reflective than film so lenses need to have better coating (at least the very last one).
Another important thing is resolution: with current APS-C sized sensors it's much easier to see differences in resolution. Back in the film days, you needed a damn good loupe or a very good film scanner, time and know-how to look for limits of your equipment. And then you probably had high-quality glass anyway
Nowadays when you want to see limits of your cheap $100 kit lens, take a picture and view it at 100 or 200 %. Done. Every bozo (no offense to all the bozos in the world) can do that, no special equipment required.
Despite all this, keep in mind that lenses are lenses. Digital lenses are not another species, they are just optimized differently. For example: you can easily correct vignetting with your favorite image editing app. Some modern cameras (at least some Nikons) do that on the fly!
Very often, moinkers such as EF-S, DX, DG, etc. just indicate the lenses have been designed for APS-C-sized (read: smaller) sensors. That makes them easier to construct, lighter and cheaper. Tokina's or Sigma's 50-135/150 mm f/2.8 zoom weighs about half of its full frame counter parts.
And of course, there is one more important point: nowadays, you can just construct better lenses just like we can buy better cars today than 10 or 20 years ago. This is also true for bodies.
Personally, I think the focus on IQ is a bit overrated these days: the lenses you can buy now are usually a lot better than what you could buy for a similar price a few years ago. There are exceptions, but overall, things have improved. I own 2 `digital' lenses and 2 `film' lenses. Of these, one of the digital lenses is clearly the `worst' (as in not as good as the others and I feel limited by it).
