Here are some of the problems you have probably experienced yourself and should avoid in order to present a good professional image:

• Ransom note. Too many font families and font changes for no reason.
• Confusion.  How do I get back where I was before? Confusing or inadequate navigation on each page. No obvious way out.
• Blah. Underwhelming. So what?
• Scrolling screens of grey text on grey background. No columns, no white space, no font changes, no color, nothing to break up long pages of paragraphs.
• Turn it off! Endlessly repeating music, flashing text, large or fast moving animated graphics, or noises each time you click an image.
• Tagalongs.  Frames that stick with you when you leave a web site.

Common sense will tell you that the sites you go back to yourself will attract others like you. Most likely the sites you find effective have these things in common that you can work into your own web site:

• Interactive and asks for your participation
• Easy to find the information you want quickly and jump right to it
• Instinctive design so natural to the site that it's transparent to the user
• A story or theme carried out from the start to the deepest level pages
• Content that you can learn from or refer to
• Easy contact with the organization or web manager on every page