Inconsistent design. Every page is a different color or shape. This is mainly you trying to get comfortable in your own site -- like trying on a lot of different clothes. However, it distracts from your content and is considered unprofessional. Concentrate on finding something nice that will look good as an overall frame for your content, and stick to that one color scheme and page design.
Broken links. Nothing will make a visitor flee faster than broken links or incomplete pages.
Disorganization. Do not confuse your visitor. Ever. Think long and hard about how you want to organize your website, and take your time with it, because visitors do not want to have to think to navigate your site. They want to find what they're looking for immediately.
Convoluted web address. I'm not saying you must have a domain or else, but I do recommend that you avoid urls that are more than 20 characters long. Although, if you can, it really is a good idea to invest in a domain name. Mine is around $120 per year. It's a good deal.
Not updating. Visitors only come back if you put out fresh content.
Music. Don't have music or video playing on your page. It annoys people who like to listen to their own music while surfing, and it takes longer to load.
Animations. Things that move draw the eye and distract from your content. This is bad. If you must share it, then email it as an attachment to friends and family -- people who have to love you no matter what you do.
Advertising. Unless you have a free host and can't help it, avoid flashy advertising. It's UGLY. If you must advertise, try to be modest and discrete. The best I've seen is Google AdSense.
Links to other sites. You might not think so, but having links to other sites encourages people to go to other sites. And I know it's good manners to link people sometimes, but . . . well, you want visitors to stay. And it's okay to not link everyone you know or like. That's what bookmarks are for. (I, for example, started my button rotation so that people would stop asking me to link to them.)
Busy backgrounds. I know sites that use really bright, colorful pictures as backgrounds because they're pretty. This is stupid because then the visitors can't read the text. Make sure your background does not obscure your words. Your words make the site.
Non-uniform text. Keep your text around the size of a 12-point font -- never make headers taller than the width of an index finger (or as high as 2 lines of 12-point font). People prefer arial, helvetica, and sans serif fonts. Avoid text that is in any way moving (blinking, scrolling, etc) or brightly colored (only use dark text for light backgrounds and light text for dark backgrounds). Also, it's a good idea to use paragraph breaks (like me) instead of indentions (like when you're writing in a word processor) when starting a new paragraph. I used to use indentions, but people asked me to change because the text was too cramped.
Being vague about the purpose of your site. The first paragraph on your home page should explain what your site is about. It's like a sales pitch. Capture the visitor's attention.
Being long-winded. You may be writing a novel, but don't make your visitor feel like they're reading one.
Bright colors. No lime green or fuscia or puce. Colors high in intensity (like neon) hurt peoples' eyes. Err on the side of caution.
Too-large images. I know that this is often the fault of not having an image-editing program. But that's no excuse. Surely, you can find someone who can re-size an image for you. Heck, if you're desperate, ask me and I'll help. (Also avoid images that don't apply to your site or are too distracting. Don't put too many images on one page.)
