As the number of computing devices available to users continues to grow, personal computing increasingly involves using multiple devices together. However, support for multi-device interactions has fallen behind users' desire to leverage the diverse capabilities of the devices that surround them. In this paper, we report on an interview study of 29 designers and developers in which we investigate the barriers to creating useful, usable, and delightful multi-device experiences. We uncovered three key challenges: 1) the difficulty in designing the interactions between devices, 2) the complexity of adapting interfaces to different platform UI standards, and 3) the lack of tools and methods for testing multi-device user experiences. We discuss the technological and business factors behind these challenges and potential ways to lower the barriers they impose.