At the airport when you weigh your bags in and are one lousy kilo over... they you are meant to feel like a criminal as you ferret around for the £20 'excess baggage' charge... only to glance over and see a 30 stone bloke sailing through bang on the limit.... although 100 kilo over on his body weight.
I've said for a long time that passenger and bag should be made to get on scales. They can take the average size of a man for example, add 25% to be fair, then add 25kg to be even more fairer, then 15kg for hand baggage to be even more fairer.
Whatever the total weight is people should be then charged per kilo (say £2.50) over.
This if course would be seen as discriminatory... not a good way to help reduce carbon footprints (assuming the overall plane weight is at least the same as now if not less) and deliver good customer service...