ILS, aerobatics, flight time, etc., these are all nice things that you can measure. The important thing -- as others have alluded to -- is judgement. Keeping the shiny side up and the greasy side down in good weather isn't hard. Hopefully, none of us would have solo'd a student if they could not do that PREDICTABLY well. The key is whether that pilot understands the limitations of his or her skill (regardless of hours logged) recognizes the hazards that will exceed those limits and assesses the likelihood of encountering those hazards, and then makes a prudent decision about managing those risks. How do you assess judgement? How do you (as a CFI) help people to develop the risk management skills upon which sound judgement is based?
If only I had taught him how to fly an ILS (although I would argue better to know how to call for help and fly vectors to a PAR)
If only he had more flight time (WWII pilots flew in combat and horrible weather with far fewer hours)
If only he had actual experience in recovering from from an accidental Lumshovak (on instruments)
We need to stress using superior judgement to avoid having to demonstrate superior skill. This is where I entirely agree with the FAA's scenario based training program. (Even though I have some serious concerns with other aspects of it.)