This question was asked by Brian Massey in a recent article on SearchEngineLand.com and I thought it would be fun to answer the question before actually reading the article.  So what might be some of the reasons that are keeping searchers from converting on your site?

1.    Content does not meet expectations.  You promised something in your AdWords ad something that your landing page doesn’t deliver.  Your meta description isn’t accurate…or maybe, your description is accurate but your content doesn’t match it.  Whatever the reason, if your content does not meet the expectations of the searcher, they will be gone.

2.    Poor design, out-of date design, or spammy-looking design.  However you want to describe it, if your site has any of these issues, people are gone.  We make split-second decisions about websites every day.  A lousy looking site or one that brings into question your trust-worthiness, will force people away.

3.    You don’t look trustworthy.  So you’re going to ask a searcher for money, or some bit of personal information.  Great.  Have you done anything to establish trust with the first time searcher? It's imperative.  People will not buy from someone (in this case a website) that they don't trust.

4.    There’s no call to action.  So you’ve enticed the searcher to your site, you’ve got great design and everyone’s Mother trusts you.  You forgot one thing.  Your call to action.  The user has little idea, without a ton of effort, what step you want them to take next.  Yes, this too will cause the user to jump ship.

These are my top thoughts, but what does Massey say is THE THING that keeps sites from converting?

Unfortunately he doesn’t answer his own question, but the information he provides is helpful.  His hypothesis is that the biggest single problem you face from weak conversions can be classified in one of five categories. 
1.    “Risk Reversal”.  Your customer is concerned about what happens after the sale.  These consumers will be looking for guarantees, easy returns and basically, a confirmation that your site can be trusted.
2.    Clear communication of your “Value Proposition.”  Are you making it clear why the consumer should buy from you? 
3.    “Social Proof.” For some businesses social media is important.  For those businesses having an easy to find, active social scoreboard is important.  There are still some other businesses for which this is not as important.
4.    “User Interface and User Experience.”  This is essentially the site’s copy and design.  I addressed this as one of my possibilities and it bears repeating.  Design is important.
5.    “Credibility & Authority” This goes back to establishing trust.  It is of the utmost importance.

Study your site to see how it’s performing in these areas and remember, whatever changes you make; make sure you test them to determine whether your changes are converting or not.