You need to make things accessible.  If a visitor finds your site via search engine, and within the first 10 seconds, can't find what they are looking for, they leave.  You need to be sure that you make what you're promoting available.

Many designers make the mistake of adding too much content (for SEO purposes) and in turn drive visitors away.  People generally don't want to read a lot on the homepage.  They want to see visually what they are looking for.  A fine balance of graphics, images, and text will keep visitors coming.

Tagging images.  So many designers add a lot of graphics and images to a site and then don't tag them properly for Search Engine Optimization.  It's not a vital part of increasing your rankings, but when you consider that Seach Engine crawlers can't see images and graphics, you need to add data to those images so that they can be "seen".

Focusing too much on all the bells and whistles, and not enough on the purpose of a website in the first place: communicating.

Links, images, Flash animation, polls, talkbacks and graphic design are all nice, and important for the site to be visually appealing, but if you don't have something to say, or you don't say it in a way that reaches your desired audience, then what's the point?

And SEO for its own sake is often a waste of resources.  Unless you are a high-profile, high-volume e-commerce site or are making money from ad revenue on your site, coming up on the first page of search results isn't going to make much difference in your company's bottom line.  I can't tell you how many clients want SEO even though their new customers don't come from the internet.

Never mind that there is now more fluff and less substance being put out there in the name of SEO than anyone would ever want to read.  If you want people to visit your site, try saying something unique and interesting in a well-written way.  If you put the information out there, you will get good results organically, because people want to hear what you have to say, not because you have the appropriate keyword density to trick a search engine's algorithm into ranking your site.

I'm hoping at some point, quality of content will again trump quantity.

Interesting points so far, as much as i appreciate beautifully designed websites, overdesigning them can b a major turn-off, it doesnt happen as much as its used to but waiting any longer than 30 seconds for something to load is a no-go for everyone but the most ardent fan of the site content. Even 30 is a long wait in this day and ageand there was research done by a company called Akamai which claimed 75% of the 1,058people asked would not return to websites
that took longer than four seconds to load.

Now that is impatient!

Other obvious ones are poor navigation, a well constructed site map should stop this from happening.

Keep it simple and won't you go far wrong.

The number 1 mistake – not having a clear, articulated plan for what the website is supposed to accomplish. As a freelance web developer, I have had a number of clients who are looking for someone to: "Make me a website." Unfortunately, in a number of cases, what they really want is a quick and easy way to make money as they've been promised by some scam. A website is simply a tool to meet a goal, and unless that goal is defined and understood, a website can't possibly be designed properly to meet it.

When the purpose is really understood, then the actual design can be done to focus on meeting the goal. Another common problem is designing a website using the latest or flashiest methods, when those methods aren't really appropriate to the goal. Just because you CAN put a 15 minute flash video or embed YouTube on your site, doesn't mean you SHOULD.

Visual appeal is important, but a website isn't a work of art. Well, perhaps some are–but a website typically has a purpose beyond sitting there and looking pretty. If a visitor can't figure out how to buy your product because he's distracted by all the flash and glitz, that's a problem. If a website has a lot of sliding menus and popups, but no up front, easily seen search function, help, and contact information–it's lost me.