This discussion has forced me to ponder my own preferences and prejudices in the genre. This is strictly a personal list. Feel free to ignore it.

1. Terminal based
2. ASCII interface available if not default
3. Turn based play
4. Still under active development (perhaps open source)
5. Complexity (you encounter something new every game, forever)
6. Difficult but not impossible for mere mortals (can include permadeath)

YMMV