While Wii Fit is legitimately huge, I think Nintendo made some mistakes which could have made it a better overal product ...
First off the Wii Fit charting and tracking should have been (somewhat) open to third party developers so people could track time spent playing DDR (for example) as cardio activity in Wii Fit. On top of this, Nintendo should have created an application for the Nintendo DS (which can sync to the Wii) where people can track their regular workouts and/or their diet on a device they can carry with them and then benefit from the coaching and charting on the Wii.
The next missed opportunity was the ability to program your workout and for Wii Fit to take you through it excercise by excercise, and to minimize the ammount of time spent in menus ... If you had the option to merge the results with the set-up of the next excercise you (possibly) get 30 Minutes of workout in 40 Minutes, which would be a huge improvement.
The last area where I think a program like Wii Fit could benefit is downloadable content ... No matter how dedicated someone is, you will eventually get bored with a workout, and with the limited number of excercises on Wii Fit this can only get worse. Even something as little as a new excercise a month could keep people interested in the game for a very long time.
One last thought ... With Wii MotionPlus on two Wiimotes, and (possibly) two ankle straps with similar technology, you should be able to come up with an aerobics like workout which gives pretty decent feedback to users which many people would probably like.
