When designing a professional looking website, consider avoiding the follow items:
Flash intros, animated mail boxes, spinning graphics, animated GIFs, etc.
Free Hit counters, which say "You are 100th visitor today" AVOID! Use Webalizer stats or similar program instead.
Try to avoid using pop up, pop under, or pop whatever boxes.
Avoid busy, confusing backgrounds, keep it clean and simple.
Always use a consistent color scheme throughout your website.
Content is king - always have fresh, interesting content. Google web crawlers love fresh and original content, thus positioning your website to a higher search engine ranking.
Autoplay music. Always allow your visitor to play music only if they desire to do so.
Keep things simple - Ultimately the goal of any website layout is to get visitors from point A to B as fast as possible. Simple, to the point, yet intuitive interfaces are the way to go.
External Links - It's always best to open external website links in a new window. That way your visitors will return to your website when they're finished browsing the external website.
Try not to go too wild on special effects - While it's fine to have a few cool effects to spiff up your website, but too many spinning objects and logos can distract your visitors from the all-important content, plus not to mention the excessively long website download times. Your visitors may leave the site even before your spinning logo has finished loading. Always design a website with the visitor in mind.