Megapixel Is the Common Denominator (Like Buying Food by Weight), 720p Sucks
You go to a supermarket and the only way to sift through the prices is to look at the cost per pound/kilogram (or for paper towels, the cost per 100 sheets).
I've started to view everything in megapixels. An iPhone displays is 480 x 320 or .153. Standard Definition is 640x480 or about 0.3 megapixels. That's assuming a 1:5 ratio (i.e. you are recording a video on a phone/camera/camcorder). Movies like Lord of the Rings are shot at 1:2.35 ratio, but then have less pixels vertically bceause it couldn't fit on a TV otherwise. Not sure how this works.
720p is actually 1280x720 or .91 MP. We use a Nikon D90 which has a 900,000 pixel 3" display or 1280x720 which is 3 times sharper per cm (or inch, as in pixels per inch, or ppi) than an iPhone. Pretty insane.
Obviously, I'm delighted that YouTube and Vimeo have so much HD content, which is actually 3 times the resolution of SD content.
But so few people dileneate between 720p (.91 MegaPixels) and 1080p, which is 1920 x 1080 or a whopping 2.07 Megapixels. Even I used to look at 720 versus 1080 and think it was just 50% more resolution - I was thinking lines not pixels. But it's actually 2.2 times as sharp (2.07/.91).
Of course, this is all going to look like silly differences when Ultra HD (2000x4000 or 8 Megapixels) comes out.
It's already out in some circles, thanks to the Red Ray player.