How many words it can understand. Making a speaker independent, continuous voice recognition system with a large vocabulary is very hard. User said one thing, system thought user said another. Substitution and rejection errors are based on confidence levels. The more important a system is, the more confidence it should have.
Most systems use synthesized speech output.
Note that audio is serial in nature. User that have become Proficient in the system don't want to waste time listening to all the options. Designers should take this aspect of novice vs. expert users into account. Monitors are curved. Smudges and dirt on the screen can lead to errors.