When the Internet came along it was free software with a Buy Now button that took you to the website where you would purchase the full version or a license key.
I thought this was the "norm" for selling software, and it always has been.
Free to try, then you must buy.
Barring a shareware style demo, I rarely make a leap of faith into a new software product. However, stellar reviews and/or at least a video example can make up for that. Maybe one exception would be updates to products I currently use.
[edited for clarity]
I was actually considering doing an experiment where I removed some of my free versions of apps from the Market and put in better videos to see if I could get the same results.... I doubt it, but...