tac7 will have the best info on Canadian cable, but just to make a quick answer: "clear QAM" is the cable-TV version of digital transmission, or you could say it was ATSC over cable. ATSC is the hi-def over-the-air broadcast standard now mandatory in the states and slowly being adopted in Canada (it will be required in Canada about three years from now). The goal of "clear QAM" was to make ATSC stations available over cable without requiring a cable box, but in reality many USA cable companies make a mockery of this by moving the channel numbers around every few weeks, confusing everyone. Regulations in Canada may be stricter which could make clear QAM more useful to you. In the USA, you're pretty much screwed if you refuse to use the decoder box. To make things more confusing, "digital cable" is not the same as ATSC or clear QAM: its just a way for the cable company to deliver more channels and pay per view features. So really, there's no way of knowing what you'll be able to receive without a box until you hook your equipment up to the cable line and experiment.
As for the rest, I'm in total agreement with tac7: any material that's 4:3 or old-style standard definition looks disgusting on LCD. Normal commercial DVDs will just about pass and usually look good, but some won't.These panels are designed specifically for Hi Def cable and satellite, and nothing else. This is the "dirty little secret" no one talks about regarding the big widescreen panels, and why many of us wish we could still buy a new 32" Trinitron instead. If all you do is watch football and network reality shows, they're great. But if you like obscure special interest cable channels or have lots of home-recorded DVDs, an LCD panel will be the biggest letdown you ever experience. The new "enhanced for motion" panels are only slightly improved, they still look like crap overall with SD sources. Like a lot of other consumer electronics trends, the consumer is not always right:they flocked in droves to LCD because its thinner than plasma, easier to hang on a wall, and doesn't have a glossy screen. But the truth is plasma is much better showing non-HD material. Too bad they're being phased out due to unpopularity: LCD panels will not come close to their performance until the excellent OLED technology drops to an affordable price.
Short-term, if you don't go bigger than the 32" size you're considering, you might be able to live with the compromises of an LCD panel, assuming you don't sit right on top of it. At 42" and up, standard-def performance gets much worse. Right now in the USA, a lot of mid-size plasma panels are on closeout sale for amazing prices, you might want to check your local stores and if you like the look better than LCD, get one while you still can.