I think that a number of important of points why Montrealers refers as Toronto as a dull city have not been highlighted. Before I list them, it is important to understand that these are "cliché", and as with any cliché, they are exagerated and long-lived even though sometimes the underlying reality at the source of the cliché has radically changed.
1) I do not know the situation today, but the bars in Toronto used to close pretty early (1am I believe) compared to Montreal (3 am).
2) Outside bars, all alcohol (including beer) is only available in specialized store (and consequently not as available) whereas in Montreal you can buy it at any "dépanneur" (and God know that these are everywhere).
3) It is true that Torontonians are typically more work-minded and Montrealers are typically more pleasure-oriented. Example: I do work often in both cities: at lunch time, people in Toronto tend to work on work premises and/or take less than an hour whereas in Montreal, people tend to go out and/or take a little more than one hour (BTW, onthe contrary, people in Montreal tend to get in at the job earlier in the morning). There are no absolutes, but these are general trends. Other example: going out to dinner with friends / family in Montreal tends to be much longer (i.e., 3 hours) than in Toronto.
I believe that these factors (and similar ones) have led Montrealers to build a "cliché" that Toronto is dull.