Poles, for instance. But even when Canada, as the US, requires visas for most nationalities, it is extremely simple to apply for one (just scan your passport, fill out the online form and pay $7 CAD). On the other hand, applying for a visa to visit the US is a stressful and relatively expensive (~$180 USD) long process with a high chance of getting rejected. This is not new though.
I'll add that, at least here in Brazil, applying for a US visa necessarily involves at least one in-person visit to any US consulate or embassy, in a business day, and they naturally don't exist in all cities. Lines are long, and the visit can take the whole day. So there's this much extra stress and expense.
