Firstly, I'd echo what people have so far said regarding quality: beware false economy.
1) Your website is the online representation of your company. A poor quality site with broken links or poorly dimensioned images will not build trust with prospective customers and will lose you countless impulse sales.
2) Be readily contactable. A perfect example of this to me is when my fiancee was booking flying lessons for her Dad. The site itself wasn't wonderful and she asked my opinion. The first thing I said was "Phone them". I wouldn't have trusted £120 to their website. However, they had a well-placed phone number on the main page that allowed us to contact them with having to hunt for it. They were very friendly and sounded legit; they got the order
3) Proper use of colour: ensure your site colours are working toward, rather than against, the site's purpose (not using blue for food websites etc.).
4) Site analysis: I don't have much experience here (yet

) but believe that analysing your site's (and individual page's) performance is an important part of improving your customers' experience.
Lastly, I suppose the general idea is: Sell something they'd like to buy, help them find you and make it as simple as they can to buy it.
Hope this is constructive.
Best wishes,
Newky.