In the summer, I have always thought Santropol is a good place for a first date because it is casual, the terrace is lovely, some of the items on the "granola" menu are often easy to share. The service tends to be slow (in this case a good thing), and it's quiet, so you can really get to know your date.
For something a little more formal and romantic, here is a list of places where you will be able to take your time and enjoy your meal, and where the food is good:
-Les Mas Des Oliviers, though when I went with my husband on valentine's day this year, the service was horrific and the food was only just OK given that the place wasn't even half full. However I hear nothing but good things...
-Trattoria on Crescent. I sort of feel like an extra in The Sopranos when I eat there.
-Fondue is always a good date food. Something about the skewers and the sharing and the little flame...mmmmmmmm. Anyway, Alpenhaus has always been good...Fondue Mentale on St Denis is a good value.
-For the same reason, sushi is also a good idea, even better for summer. Shodan on Maisonneuve maybe?
-BYOBs are always good too. I like Petit Plateau because the food is really very good and the ambiance is warm and cozy. It's a restaurant I associate with colder climes though so maybe not really summer fare.
-Concept places like Dual or Robin Des Bois are good because they are conversation pieces (though let it be said that I have been to neither of these places. For shame!)