Probably the ability to see a sentence written in spanish, and be able to click each word so that I can hear how it should be pronounced. Needless to say, those voice samples should be from a native speaker.
Practices - ie question and answer sessions. These should be based on individual lessons, or usable in a mix-and-match randomised fashion, so that you can mix-and-match questions relating to various lessons. The idea is that you can (for example) run past and future tenses together in the same test, to ensure you've really got a handle on which is which.
Comprehension: many systems focus on pronunciation & sentence construction to the exclusion of comprehension. It's no good being able to ask questions if you're not going to understand what's being spoken back to you. You don't always pick this up in a classroom setting.
I'd be happy to assist in alpha/beta testing if you're looking.