Limited User Interaction: SMS applications are a perfect fit for simple query-based services. People are entering one or two keywords and get the answer. Given that there is no direct interaction, no online-help of any kind, people have to remember the keywords and the format to enter them, and so the list of these keywords has to be limited. Complex multi-cycles interactions in such stateless environment are almost impossible to implement. The availability of only “raw” text (no style or decoration techniques available) could also be a problem to attract user’s attention on important points.

W3C on the mobile web