It all boils down to these 4 important factors to achieve a good Google Adwords Quality score.
1. Clickthrough Rate = Clicks / Impressions
2. Ad Text and keywords
3. Landing Page Relevance
4. Landing Page Load Time
And just remember that your Cost Per Click (CPC) bid has nothing to do with your Quality Score.
The first 3 has always been the golden rule of adwords quality score. Landing page load time is quite new as it was only announced in June 2008.