Hi Tom,  I do a lot of work now in researching and writing content for websites.  The most common mistake is not to treat a website like an 'employee'.  You must give it a proper job to do, oversee it regularly and make sure it's earning its keep.  Gordon is absolutely right when he says lots of businesses think it will boost sales without effort.

It should market/explain your product or service clearly - focusing on the benefits to the customer and the problems they have that it could solve.  It should have sufficient detail and comfort zones to build confidence in the buyer/enquirer: full address (no p.o. box numbers), contact names, telephone and fax numbers, email addresses.  Any accreditations and trade associaton memberships, case studies and testimonials from happy clients.  

Also because of the recent changes in search engine algoythms - VEO - visitor enhancement optimisation - you should put free stuff on there.  Add a 'knowledge centre', beneficial and interesting information for visitors to use whether or not they contact or buy.  This increases stickiness (return visits) and also if you have it in content management format so that you can edit the text and some of the images (but not the overall page design template), you should make regular changes and additions to the main body and especially the knowledge centre content.  This gives the search engines a little dig in the ribs and alerts them to changes, which means they will revisit and rerank your site as being helpful to visitors.

I now spend much of my time on pure research for clients: recently for a heating and air conditioning company I found articles from Personnel Today which related research on tempers fraying in hot weather and recommending portable air conditioners - credible source and a great sales point!

Hope this helps you.  I have a Masters in E-Business where I focused on business strategy on the Internet and Internet Law: I am an internationally published business academic - not bragging but just to reassure you that I sort of know my stuff.

Janeyp  M.Sc.

Many thanks for input - always interesting to get other opinions