No matter how hard we try to instill or implement awareness of these things developers still continue to make them. Here are the ones I have encountered in my career as well as those we've already seen others make.
Some Terms:
Customer/client: The owner of the website. (this is from the point of view that YOU are the developer.)
Visitor/user: The customer's customer. These are the people who will generate revenue and traffic for the website.
1.) Thinking that having a website is cool.
No goals, no direction and no point. My former boss, a marketing graduate from the University of Wisconsin once said that a website is an amplifier. If your business and products suck in the real world they're gonna suck much, much more on the Internet. Straighten out your business before going online. Websites have to be "defined" out of a need and not made on impulse. It is a personal business possession that requires just as much planning as you would in the planning of your products.
2.) Not understanding the customer's requirements.
Ok, most of the time we don't make this mistake but it is still worth mentioning. Wrong translation and analysis of your customer's requirements will lead to waste of time, missed deadlines, and even worse, loss of the customer's confidence in you.
3.) Not understanding the technology to be used for the project.
If you have this kick-ass idea or technology to apply or try out on a project, make sure you've studied and tested it beforehand. Some technologies may not be practical in all applications (of course, we experienced developers know this but let's prevent the newbies from making our mistakes). Some technologies or practices may be too complex to maintain for a small-scale site or it may be inflexible for a large one. Nevertheless, failure to weigh things in your project and anticipate change is a development and maintenance nightmare. I admit I lose sleep over things like these in my projects. No customer is a scratch paper for your eccentric experiments! It will ruin your relationship with them and most of all screw up the entire project.
4.) Poor marketing.
Back to basics on this one. While your customers may not care what technologies and skills you are capable of doing, marketing staff want to know the nitty gritty details in plain English (I know that sounds like an oxy***** but believe me when I put it that way!) Remember during our college thesis days we have to present and defend our work. Marketing staff are there to help you and your customer (their boss) sell your website. But in order to do that, you have to present and defend your work to them so that in turn THEY will present and defend your site to visitors.
5.) Poor usability.
The site is hard to understand and get around - most probably due to poor design or OVER design. As what my past manager once said: "Never trust an engineer to explain our products to our customers! That thought alone is unsettling."

I agree with this and at some point in the past I was guilty as well. Design your site as if YOU were the visitor. Or even better, design your site while at the same time imagining that it is your rival colleague's work. Doesn't make sense? Well, knowing that we developers really love to scrutinize our rival's work imagining our work on the hot seat really sharpens our ideas and puts our work in focus. Basic idea? Make sure the the site gives the visitor the information he needs ON INITIAL PAGE LOAD. People tend to quick scan web pages as it comes up on the search engines. If the keywords they are looking for is not on the page they clicked, you can bet they left in a hurry. Make sure when your site loads it catches their attention and contains text they were looking for. The visitor must also be able to get around the site with ease. If you still border on making extraordinary web designs, albeit kick-ass layouts and graphics, anticipate learning curves for visitors - it must not be too steep (no more than 15mins to learn). Lastly, HELP must be available. Document your site well and in plain English. I need not emphasize that technical jargon is totally useless to visitors.
6.) Feature saturation.
My personal favorite and surprisingly the most common but VERY AVOIDABLE mistake done by web developers. This is like the combination of 2,3 and 5. All of the companies (except the first one) I have worked for in the past made this mistake. It's when developers and artists throw in every trick in the book to make a site as flashy and feature driven as possible. It's the "Oh, look we can do this! Let's put this in our site! Oh, look we can do this too! Let's strap that in as well!" syndrome. While having endless features and a cool looking website are all pleasant to a customer, their visitors, and most definitely US, it almost always obscures the project's design goals. Sometimes we get so overboard with our skills that we lose our focus in what the customer wanted. All the years I have worked on websites only two things are certain: a) the visitor wants too see and know the products, and b) the customer doesn't care what you are capable of doing (and this is because most of the time they are not technically inclined). Everyone wants the site done ON TIME and WORKS AS EXPECTED. Hell, even the customer or visitor will appreciate a site with simple text and no css as long as it lists the products they are selling and visitors can read information of said product! Simply put, what comes out of this mistake is everything you'd expect to go wrong in a website. 1) It will not scale properly due to the numerous features and plugins from different developers. Oh, yes! We know this will happen! Something breaks with every update! 2.) Visitors get lost. Even worse: They can die of starvation being lost in your site for days!

3.) Maintenance and bug fixing will take days.
LASTLY, your job as a web developer is just the same as any IT professional. While the customer has the last say and he may ALWAYS be right, his ability to be right will be based loosely on how you well you give him the ideas. You're here to provide a SOLUTION for your client. If you feel he is making a mistake bring it up. Defend it!
Feel free to contribute.