Here in Europe airlines have weight restrictions for carry-on luggage - some are more generous than others, but for most airlines it's between 6 and 10 kilos. The only exception to this is Easyjet which has recently abolished its previous 6 (?) kilo limit in favour of a rule which says " You can carry it on if you can lift it into the overhead bin unaided".

Weight restrictions do get enforced - check-in agents won't weight every single piece of hand luggage, but if it looks heavy, or if you're not checking anything, then you will be asked to put it on the scales, and if it's over the limit you get the option of checking it or transferring heavier items into your hold luggage.

Why don't North American carriers introduce weight restrictions?