Primarily, the court found that she could not be adequately supervised and could not perform the 'teamwork' aspects of the job, such as covering for fellow employees in what she recognized as a sometimes hectic work environment. One thing that I was proud to see the court do was strike a blow for common sense, and for those of us who were somewhat appalled by the rush to add "regular attendance at work" or some other such phrase to job descriptions in the early days of the ADA.
In the money quote the Court held:
Here, the only evidence Mason proffered in support of her argument that she could perform the essential functions of her job from home, other than her own self-serving testimony, was the absence of attendance, supervision, and teamwork from the service coordinator job description. We are not persuaded the absence of those functions from the job description demonstrates those functions were non-essential. As common sense suggests, Avaya probably did not even consider informing its employees that they were actually required to show up at the workplace and work with co-employees under supervision when it drafted the service coordinator job description --- that is a given. Consequently, we find the omission of physical attendance, teamwork, and supervision from the job description entirely unremarkable.
The Court, wisely given the guidance of the Supreme Court, did eschew any per se rule against at home work as a reasonable accommodation, but also made it clear that it would be the rare situation where a plaintiff could demand it.