It was needed because the ADA as it stands currently protects only people who are "substantially limited in a major life activity," or people who have a record of or are regarded as substantially limited in a major life activity. While this may sound fine, the Supreme Court had interpreted "substantially limited" and "major life activity" really strictly, such that in order to be substantially limited in an activity you had to basically not be able to do it at all, and a lot of lower courts were interpreting "major life activity" as not including communicating, thinking, concentrating, and interacting with others. This really hurt people with mental disabilities because those same courts would, for example see "speaking" as a major life activity, but not "communicating" or "interacting with others" (ableism, much?) so people who could talk but had serious social interaction issues weren't seen as disabled. It also hurt people with epilepsy, because hey, if you're unpredictably unconscious for a few seconds every day that's not substantially limiting because 99% of the time you're not unconscious, and people who mitigated the effects of mental or physical disabilities with medication or auxiliary aids or compensating techniques.

Overall, a whole lot of people with autism or asperger's would have found themselves excluded by the old ADA. I don't think the new version is perfect, because I would have gotten rid of "major life activity" talk altogether and they didn't include social interaction as one of the examples of major life activities (they did include thinking, communicating, and concentrating, and courts are still free to decide social interaction is also in there). But I do think it's a lot better. The House passed this bill by a landslide, which is great.