This is really just a thought experiment that popped into my head.
Let's say you have a product that interacts with the user via a screen. The screen cannot display graphics, but can display text in a variety of ways (think ncurses, and the curses development kit, or otherwise just plain text). Now, a lot of interaction design goes beyond user interface. You have to get the user requirements, work out what information the user needs to be exposed to and at what time, and all sorts of other things. The issue is, however, we live in primarily GUI world. MP3 players, for exampled were heavily text-interface-based, but now have largely moved to a graphical interface. There are always things like embedded computers, of course, or esoteric systems like Gopher, or command-line power users, but these are niche markets.
Assuming you had to develop a mainstream product that featured a text user interface, how would that impact your design process (if at all), and how would you change how you think about it, or otherwise compensate?