If the website say, for example, allows users to download a super important PDF file, then it should do it well. It should do it over and over again without fail. It should also be easy for the visitor to figure out how to perform their part of the task. This may sound like the obvious, but all to often I go to a site that has the potential to be a great one, but then they drop the ball at the crucial moment. So, test, test, and test some more.
How a website is organized, the nature of the site's navigation, and how visitors will interact with the website are all part of something called Information Architecture or IA. This is one of the more crucial parts of a website's design. If your visitors can't find what they need, then they will simply leave and never come back. Great sites understand the visitor and organize their content in such a way as to make it as intuitive for the visitor as possible.
Style and design aspects are what most people think of when they imagine how cool their new website is going to be. This is the emotional component of a website. Much like what a person is wearing, the type of perfume or cologne they may have on, their style of haircut, do they have on a Rolex watch or a Swatch, leather and beads or gold and diamonds. A great website makes the visitors feel at home. It will make them feel comfortable and at ease. This can only be done if you design your site for the appropriate audience. If your website targets teen girls, use common sense and don't use a conservative business motif.
A slow Load time for a website is always a sure way to lose a visitor before they even get to your website. They teach this in every basic website design course from here to China, but getting that fat content out there is just too tempting. Video and bump'n Flash animations are all the rage, but they have to be used where and when appropriate or they will cause more harm than good. A great site has a great load time.
A lot of quality information is always at the core of any great website. Without good content why would a visitor even want to come to your website? However, having exceptionally unique and valid information will set it apart as an exceptional site.
How current and up-to-date a site's information is can make or break it. Exemplary websites will always be 100% current and be used as an almost real-time communication tool to interact with their visitor base. Stale content, even if it was all the rage last month, often times just won't cut it for today's best websites.
Making sure your visitor's time at your website is as safe from hackers and criminal elements as possible is vital to any website. There is no doubting that your website sucks if someone gets their identity stolen while visiting. This can mean spending some money and making recommended changes to otherwise working code, but it has to be done. Security for your website and the people that visit it is critical. Call in the experts and play it safe.