I like being able to select a number of possible departure airports (Ottawa, Toronto, Montreal, Detroit) and then my days I like being able to say +/- 2 days.

You can do a lot more than that in Google Flights, just hit the plus sign on the right side of the departure box and you can add as many airports as you want, ditto for the arrival airport. Also, for the plus or minus 2 days just click on the little bar graph at the top right of the page instead of map view and it will show you months on either side. That feature is probably one of the best, there is a huge difference in prices depending on the date.

Matrix and ExpertFlyer also let you search fare classes, so if you know your upgrade instrument is based on the availability of X inventory, you can look for flights with available X inventory so you can upgrade.

For me three features are missing in Google Flights: 1. International flights 2. Flexible dates (simple +-3 or the whole month overview) 3. Ability to specify connecting airport