Since my friend Alex Day (yep, that’s a callout) seems to think the user interface of Nethack is horrible and a detriment to the game design, I have naturally taken the opposite position. The rogue-like interface, as part of the difficulty of these games, seems to me an intentional choice (especially as newer rogue-likes decline to update the UI). Learning the interface is part of learning the game (in any game), and in this case the choice has been made away from usability (which is why Alex hates it). This choice makes sense, however, in light of the rest of the game mechanics, which involve learning what actions are possible and what strategies are useful through trial and error as well as dumb luck. The fact that a single keystroke can ruin hours of play is part of the fun of rogue-likes, whose difficulty and complexity are the key elements distinguishing them from action-RPGs like Diablo or older traditional RPGs like the Gold Box (Eye of The Beholder, etc.). It is easy to imagine a rogue-like with a “nice” user interface, and I won’t argue that such a game would not be fun. But to many fans of the genre, it would feel lacking.
There are many genres where a usable user-interface is a requirement, but there’s a difference between a user-interface that is bad and one that is merely difficult on purpose. Every PC shooter fan (myself included) is quick to point out the superiority of the mouse over the console controller’s joysticks for aiming. Certainly PC shooters would be more difficult to play with a console controller. But this controller joystick, when used properly by game developers, becomes an asset in making simple but fun shooters such as Halo (okay I said it, Halo is fun). Maybe I’m stretching the definition of user-interface here, but I think you get the idea. Learning a new way of interaction can be fun (Wiimote, anyone?) or challenging, but either way the difficulty of the UI is a design choice that can be in some cases exploited for the benefit of the game (and thus it’s players). The prime example of this, in my mind, is Steel Battalion. Just google a picture of the controller and user-interface. Footpedals (3 of them)!
Is your user interface designed to be merely functional or as good as the competition, or are you really adding to gameplay with the capabilities it offers? Most of the time the former camp is what you want, but there's always room for experimentation.