Americans that live in Canada now have to file a tax form and pay taxes every year in the US. This wan’t always the case but it has been in place for several years. It has resulted in some dual citizens giving up their US citizenship for financial reasons which is unfortunate.
I believe that if you have no US investments or property it is more of a flat tax, otherwise it can get complicated and require hiring tax accountants in the US which can get expensive. When you didn’t have to do this in the past it can be a shock to the pocket book.
If you're a US Citizen your worldwide earned and unearned income is subject to tax in the US. You can however file form 2555 which is the Foreign Earned Income Exclusion. This is been the law for a long, long time. I use to work in Saudi (for a non-US employer) and use to have to file this every year. You can take a foreign tax credit which is often more favorable than taking the earned income exclusion. If you're working in Canada it's even easier to file as you're subject to Canada-U.S. Income Tax Convention of 1980. Just have to make sure you understand the rules and live by them. There isn't anything drastically new.