As a web designer, you should design your websites to give your visitors the greatest ease of use, the best impression and most important of all a welcoming experience. It doesn't matter if you had the greatest product in the whole world -- if your website is poorly done you won't be able to sell even one copy of it because visitors will be driven off your website by the lousy design.
When I'm talking about a "good design", I'm not only talking about a good graphical design. A professional web design will be able to point out 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.
Hence, I have highlighted some features of the worst web designs I've 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 should know it's high time to take serious action!
1) Background music
Unless you are running a site which promotes a band, a CD or anything related to music, I would really advise you to stay away from putting looping background music onto your site. It might sound pleasant to you at first, but imagine if 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 turn 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 just to view your site as it is meant 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 part of it too! You should design the text on your site to be legible and reasonably sized to enable your visitors to read 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 display 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 do have one like many users out there!) and, well, pops up on my screen. Imagine if you had a very important message to convey and you put it in a popup window that gets killed most of the time it appears 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 website does what it's meant to do effectively. Don't let some minor mistakes stop your site from functioning optimally!