Boy, this is a tricky one. The rules for entering Canada haven't changed, but those for the US have changed. So I will try to do this as clearly as possible.
Canada does not require a visa for American citizens. In fact, Canada just asks for proof of American citizenship and government issued photo-ID. (But you aren't off the hook yet... you need to worry about US law).
US law requires you to present a passport if you are flying between the United States and Canada. (It's not a Canadian requirement.)
If you are crossing the border by land or sea you are required to have either a passport or an enhanced drivers' licence. Mostly border states/provinces are working or have issued them. They are special licences that have an RFID chip and you would have needed to provide proof of citizenship to obtain the licence. These are acceptable only for land and sea crossings.
The US passport card is not acceptable for flying, only for land/sea and only for entry back into the US. Canada accepts this as government issued photo ID and proof of US citizenship, but the document has no legal international standing because it is not defined in International law. No country is legally required to accept it at all.
One further note that many people forget. Those with a criminal record (with a criminal offence, not a summary offence) are generally inadmissible to Canada without a visa waiver, unless they are considered rehabilitated (usually 5 years with a clean record after the sentence.) Remember that this is defined by what is a criminal offence in Canada and that includes driving while impaired, even if adjudicated in the US as a misdemeanour.) This is one of those tit-for-tat things... the US government does it, so the Canadian government does it in kind. Information on obtaining a waiver are on the government of Canada website or you can call 1-800-OCANADA.