A generally acceptable amount of time for a user to wait for certain system responses such as an application launch or switch.
The theory that users will abandon a website if they are unable to complete their task within 3 mouse clicks.
Section 508 of the Rehabilitation Act was enacted to eliminate barriers in information technology, to make available new opportunities for people with disabilities, and to encourage development of technologies that will help achieve these goals. Under Section 508 (29 U.S.C. § 794d), Federal Agencies must give disabled employees and members of the public access to information that is comparable to the access available to others.
Based on the Pareto principle, applied to any website, web app, or software environment tells us that 20% of the functionality and features in any one environment will be responsible for 80% of the results, or actions taken within that environment.