The visibility of the new tax was one reason, but I’d suggest that opposition was also partly due to the previously invisible federal sales tax (MST) being applied (as GST) to things it had never been applied to before. Newspapers bore prices like “50c (47c + 3c GST),” barbers and taxi drivers and parking lots had to add GST to their prices, and postage stamps were taxed. Professional service providers–lawyers and accountants and similar–now had to add GST to their invoices. Such things had never been taxed before; now they were.
The GST was promoted as “it applies to everything,” but there were a number of exceptions, which created confusion (and thus, opposition) among consumers. Just what was GST-only, and what was subject to GST + PST? And was there anything not subject to GST? All provinces (except Alberta) had sales taxes, but they only applied to certain things; certainly not everything. In Ontario, IIRC, books were not subject to provincial sales tax (PST), but were subject to GST. Similarly, feminine hygiene products were exempt from PST, but were subject to GST. Groceries were to be exempt from GST, but you had to buy at least six muffins for it to qualify as a grocery purchase; buying a quantity of five or fewer was deemed to be “for immediate consumption,” and they, like a few muffins purchased at a coffee shop, would be taxed. Eye exams and corrective lenses (and any medical procedures and prescriptions and prosthetics, etc. not covered by provincial health plans) were not taxed at all, but the frames for your new glasses were subject to GST, and possibly PST. I’m going on memory here; I may have a few details wrong, and I know some of these examples depend on province, but I hope I’m illustrating the confusion that arose out of the fact that federal and provincial sales taxes did not always mesh perfectly.
Drivers and drinkers and smokers were hard hit, as now there was a fourth tax on gas, alcohol, and tobacco, respectively: federal excise tax, provincial excise tax, PST and GST. There was a worry that the GST would be applied on top of the others–effectively a tax upon tax, in other words. This was, naturally, a concern, which caused some opposition. (In the end, the GST turned out not to be a tax upon tax.)
Lastly, Canadians were informed that the introduction of a GST had done wonders for the economy of New Zealand. With all due respect to the people and government of New Zealand, Canadians wondered why we were emulating a country that had no sales tax of any sort before their GST, when we had been paying provincial sales taxes for years. Don’t the feds understand that unlike New Zealanders, we’re already paying sales taxes? Why do the feds want to make us pay their sales tax and our province’s sales tax? We’re not New Zealand, with one level of sales taxes; why are you treating us as if we were? Such questions were answered with (to many Canadians) inadequate responses from the federal government, which only fueled opposition.
As I recall, the federal government did a very poor job of explaining why this tax was being implemented, what we could expect from it, and how it was going to be implemented. In retrospect, it turned out to be a good thing; but at the time, with a poor explanation of all of the above, along with the consequent rumour and conjecture that arises any time something is poorly explained, it is no wonder there was opposition.