A good impression
When a visitor loads your page you want them to be impressed by one, how your site looks, and two, the functionality of it. If they don't like it, they will simply hit the back button, and go to your competitors website looking for better information, and a better design. You want your website to look good the first time the page is loaded, so it gives the visitor a good impression.
If you are selling a product, you want your site to give off a good impression from the start, so the visitor likes it, and continues to browse the website for what they are looking for. You want your website to be professional in every way, shape and form. I would much rather find a website selling the same item, with a nice, professional design, and buy from them, than buying from a website with a bad looking design, but with the same item. If you do not have the knowledge on how to design your own web page, than there are many web designers out there that will do it for you for a good price.
The functionality of your website plays a big role. Make sure you have a easy navigation system set up. I, for one, will usually leave the website if I know there is a page that I want to get to, but I cannot seem to find a link anywhere on that website. Make sure you have a link on every page the points back to your homepage, and on the homepage, a link to every content filled page on your site. If you are selling a product, make sure it is very easy for users to find the product they want in a search, and then read about it. There are many 'shopping cart' scripts out there, some even free.
This may not be one of the points I listed in the first paragraph, but it is also a very important one. If you are selling a product, you should always make clear what you are selling, and why they should buy from you, and not some other company. Sometimes a splash page can accomplish this, but in some cases it will just annoy your visitors. A good way to do this is on your home page, in big bold letters, say what you are selling, and why they should buy from you. Be careful to not get carried away though, sometimes you may say something that will turn off a potential customer.