1. Proper instruction according to a mandatory syllabus by Instrument Rated instructors
2. Appreciation of weather related issues.
3. Appreciation of the IFR system.
etc etc.
Until you do an IR you don't know what you have missed by doing just an IMC.
True, as Bose X says there are some competent IMC PPLs around who use the rating for more than a get out of jail card. They are however a miniscule fraction of the total of people with an IMC.
It is however a bit unfair to make out everyone with an IMC rating out to be a cowboy ready to fly beyond the limits of their abilities and rating. The lack of IMC related accidents by IMC rated folks proves that in my opinion.
FD