From my understanding, you should be fine as long as you don't have an arrest warrant against you, you don't land on US soil or your name is not flagged up by Interpol for serious crimes (being a sex offender, a drug trafficker or murderer for example). Having a pardon is a plus because your conviction is erased from CPIC and if other countries ask for info, they see nothing. That being said, your info may have been given to Interpol even if you get a pardon especially when you have an active warrant against you.
The US is different because they have full access to CPIC without Canada's permission, getting a pardon does not help here. Having full access to CPIC means that your criminal record after receiving a pardon will show as sealed. The Americans may have also downloaded your record before you received your pardon. There's a lot of things governments and police forces don't tell us. You can thank Harper for giving them full control, but I'm pretty sure any other ruling party would have done the same. Since 9/11, North American society is driven to punish people for petty crimes while the true criminals seem to get all the help to restart. A lot of civil liberties were thrown out and many people are fighting to get them back.
That being said, the 72 hours requirement is only required by DHS (the Bush administration's idea). George Bush Jr. himself had a record, but he became president.
The US tried to convince all countries to participate, but a lot of country simply did not cooperate. You have too understand most South American countries rely heavily upon Canadians to boost up their tourism industry. The US government does not realize it's harming it's own industry. Applying for a waiver is nothing more than a money grab and many people refuse to go to get them for this reason.
That being said, there's always a risk, so be prepared. Take a direct flight that avoids landing in any US city and make sure that your name is not on Interpol's database.