A bike is "too heavy" if you can't carry it up stairs (and you must do so).
A bike is "too heavy" if, with the stock gearing, you're unable to climb the hills on your commuting route.
A bike is "too heavy" if you find that its weight prevents you from completing your commute in the time you have available (yes, that is entirely subjective).
A bike is "too heavy" if you find that its weight prevents you from enjoying your commute, and you commute strictly for "enjoyment."