In answer to the question of how many themes are ruined: most of them!

Sorry, but for me this is a fact. The small thumbnail looks good, it makes people want to click on the demo, and then all that comes up is a plain theme with no excitement which hardly inspires and which looks nothing like the thumbnail. I spent over an hour recently looking through themes to find a suitable one for a new web site (http://vmpublications.com) and rejected almost all of them simply because the demo didn't include header images or had only the one boat photograph, no decent text to show typeface and layout and gave no idea whatsoever of what a theme would look like when actually used.

Even non-commercial themes need to 'sell' so a good deal of improvement is desired to gain more downloads. We can't all download a plain theme and work on it to see what the creator should have shown us in the first place.