When we last visited Canada almost four years ago, very few merchants would accept my Discover Card. Is that still true?

Yes

Are American Express, Visa, Mastercard equally as popular?

A wide variety of merchants accept Visa and MasterCard. Fewer merchants accept American Express.

Thanks to you both for such quick answers!

Most restaurant and hotels/motels accept Amex and about half of those also take Diners but you can be sure that 99% will take Visa and/or MC

What about Visa/Mastercard debit cards?

Canadians use debit cards profusely-no worry on that front.

Thanks

You can use your debit card just like a credit card everywhere, and in the ATM's.

Note that you must look for your logo (Mastercard, VISA) or sometimes the logos on the back of the card (Cirruss, etc.) when selecting an ATM. Depending upon where you are in Canada, often some take one but not another, and their are many local banks not on any international network at all.

If you have leftover currency, use it toward your hotel bill so as not to have to change it back.

Good point on the Cirrus versus whatever. I ran into that situation in Lake Louise trying to use my ATM card for cash. The machine I found there is a private concession and it would not take my cards, none of them. I am PLUS; it was Cirrus. Besides, it wanted a fee. So stock up on cash elsewhere. No problems with an ATM card in Calgary or Edmonton.
Don't know about Banff; did not try there.
Farther up the road at The Crossing, I ran into the same problem. I was the wrong breed for the money machine.

Bob has a good point about the "private concession" cash machines - avoid them at all costs! The cash machines that do have better card options are all linked to Canada's major banks: BMO (Bank of Maontreal), CIBC, TDCanada Trust, ScotiaBank. Look for one of those names on the ATM.