As a web designer, you should design your websites to give these potential customers the greatest ease of use, the best impression and most important of all a welcoming experience. It doesn't matter if you'd the greatest product in depends upon -- if your website is poorly done you won't have the ability to sell even one copy of it because visitors will be driven off your website by the lousy design.
When I'm discussing a "good design", I'm not only talking about a good graphic design. A professional web design will be able to explain that there are many components which contribute to a good website design -- accessibility design, interface or layout design, user experience design and of course the most straightforward, which is graphic design.
I've highlighted some features of the worst web designs I have come across. Hopefully, you will be able to compare that against your own site as a checklist and if anything on your site fits the criteria, you have to know it's high time to simply take serious action!
Unless you are running a site which promotes a band, a CD or any such thing related to music, I would really advise you to stay away from putting looping background music onto your site. It might sound pleasant for you at first, but imagine in the event that you ran a big site with hundreds of pages and everytime a visitor browses to another page on your site, the background music starts playing again. If I were your visitor, I'd just switch off my speakers or leave your site. Moreover, they just add to the visitors burden when viewing your site -- users on dial up connections will have to wait longer merely to view your site as it is intended to be viewed.
2) Extra large/small text size
As I said, there is more to web design than purely graphics -- user accessibility is one big element of it too! You should design the text on your site to be legible and reasonably sized to enable your visitors to learn it without straining their eyes. No matter how good the content of your website or your sales copy is, if it's illegible you won't be selling anything!
3) Popup windows
Popup windows are so blatantly used to show advertisements that in my mind, 90% of popup windows are not worth my attention so I just close them on instinct everytime each one manages to pass through my popup blocker (yes, I really do have one like many users out there! ) and, well, pops up on my screen. Imagine in the event that you had a very important message to mention and you put it in a popup window that gets killed most of the time it seems on a visitor's screen. Your website loses its function immediately!
In concluding this article, let me remind you that as a webmaster your job is to make sure your internet site does what it's designed to do effectively. Don't let some minor mistakes stop your website from functioning optimally!