Not many people know this, but Adwords also looks at your website bounce rate and factors that into your QS. - this isn't necessarily true, there's a lot of discussion about this - for this to happen though you'd have to have analytics on there and not everyone does - so I'm not too sure on this at all
so what do you think is more important keyword density (is the a min word count) bource rate use of h1 or h2 tags
MOST important asects of QS in this order: 1) keyword in title of ad and display URL 2) keyword in landing page 3-4% density ( no word count min) 3) "about us" page and sitemap page 4) If emails are collected, then "privacy page" is needed.
For our landing pages we have a script that will dynamically insert the search term/phrase into the title tag and H1 for visitors that arrive via ppc which seems to help with quality score and conversions. i.e.. "Looking for [keyword]? Consider [product]."