Most European carriers have weight restrictions for carry-ons (I am only aware of ONE that doesn't - Easyjet).

Most, if not all European carriers use the weight concept rather than the piece concept for checked baggage.

This means that unlike American check-in counters European ones are equipped with scales - making weight restrictions quite easy to enforce. (I was quite surprised the first time I checked in for a flight in the USA and found there were no scales at check-in...)

All checked baggage gets weighed at check-in, and there are weight spot checks on carry-ons - if something looks heavy they'll usually weigh it, if it doesn't they usually won't.