It seems like much of the accessibility debate has centered around "all or nothing" issues, like text to speech for the blind etc.
But very little attention has been paid to what I guess you could call "marginally disabled" or "visually challenged".
For example, a lot (billions!) of people do not have perfect eyesight, or perfect color vision, yet time after time I see sites with 6px sized text, blue on blue navigation links, and similar.
IMO, accessibility designing should include all of those factors to some extent.
And just for information, here is an excellent color blind test for web pages - try it out on a few and you might be surprised. I found a couple of popular sites that are almost unreadable for some common types of color blindness.
I ended up adjusting a few colors on our main site using that.