I go every year, sometimes twice. Here is why:
the People - entirely sane and reasonable. Friendly and relaxed without being phony about it. Honest and expect the same of you (the token booth clerk for the subway will click you through if you are going to UQAM to buy a pass). I have had Canadians tell me that THEY find Montrealers a notch "stand-offish" and they attribute that to the french influence. I didn't see that.
Jean Talon Marche - big open air green market in the Petite Italie section of the plateau. Gorgeous local produce and free samples.
Sunset from the top of Mont Royale - take the bus up, hang out for the golden hour then walk down into the city.
the Sunday drum circle - about 150 locals pounding every type of drum. It gets old but worth a stop. Off of Avenue du Parc.
their subways - quiet rubber tires, high ceilings and they make this really cool high tech sound (like the land speeder in Star Wars) when you ride them. Their subway beats New York, London and Berlin.
the Food - Montreal has more restaurants per person than New York and probably every other city in North America. The competition means only the best survive. Add to that, Montrealers are not 'afraid of food' (as Julia Child said of Americans). Cheese, bread, desserts, game, poutine, beer and wine. And to cap it off -- they are thrifty and value oriented - you can bring your own wine to many places.
the Firework competition - every week during the early part of summer, there is a fireworks show by one of the international teams which compete. Half the city turns out; young couples, families, tourists.
the whole island thing - I love cities on islands (coughing: '...manhattan'). There is a stronger pride of place perhaps because of the physical boundry. It is as if the separation gives a city the extra impetus to have a strong personality.
Miscellaneous - $1 pizza (thats 83-cents US), Tim Horton's donuts, Au Petit Extra;
http://www.aupetitextra.com/ , Le Grand Comptoir (1225 Phillips Sq), a big glass of Boreal Rousse, smoked meat sandwiches, french fries with flavored mayonnaise, Vieux Montreal, the casino (just don't gamble), Chinatown and the way they speak french.
See you there!