Here are my thoughts in no particular order:
1) Optimum distance: 6 - 10 miles. At an average 4 minutes/mile pace this gives you the recommended 45 - 90 minutes of daily aerobic exercise (up from 30 minutes recommended years ago).
Secondly - these distances are short enough to be competitive with other modes of travel.
2) Route should consist of safe roads/neighborhoods. A real plus if the route is scenic! Nice if you have the option to vary the route to avoid "route burn-out".
3) Places where you need to do daily errands are along your route. Think grocery store, video store, library, etc. You don't want to have to ride home then get the car out to take care of things.
4) Secure covered/indoor parking for your bike.
5) Shower & Locker room facilities.
6) An alternative means of getting home should weather or work take a turn for the worse.
Over the years I've had commutes of 23 miles, 17 miles, 16 miles and now 10 miles. For me - getting the distance down to 10 miles was key in being able to commute daily (and go car-free
) rather than commuting sporadically.