Part of my job is to assess if websites are usable for people with disabilities. Over the past year I have audited many large-scale web experiences across various industries. I've helped three airlines in China and Canada and some Canadian retailers. I've assisted some US educational institutions.
Through these audits I have identified the top 5 most costly website accessibility issues. These are the ones that have the most detrimental effect for users, and that will take the most time to fix later.