1. Different visited link colour rather than just having all your links the same colour it’s just annoying for visitors if you don’t have visited colours.
2. Fully flash sites big no no, if you’ve notices that flash player has only just been released on Linux so a lot of them won’t be able to see your site or some windows users still don’t have flash player so avoid flash at all costs.
3. Do not design over 800×600 this is obvious I hope I don’t really need to explain it.
4. JavaScript do not use a lot of it and don’t depend on it. You will find that your visitors may have it disabled.
5. Do not use Tables, if you’re a web designer you should already know this for your layout never use tables as it slows your users loading time down css layouts are a lot faster with a external sheet style.