As a website user and designer, I have a few pet peeves when it comes to website design.
#1 Flash
Although they seem "cool" to the site owner, Flash sites can be extremely frustrating from a user standpoint. I can't really think of a reason to use Flash, except to have that "cool" effect. Almost anything you hope to accomplish with Flash can be done (much better) without it.
#2 Music
Another "cool" feature to many clients is music, but it can be a big turnoff to site visitors. I can only think of one site that I actually allowed the music to play (a photography website while the gallery was playing through), and I stopped it after a few minutes. Music is a very subjective thing, and its effect can change with the mood of the visitor. When in doubt, leave it out.
#3 Splash/Intro Pages
In the same vein as Flash sites, splash/intro pages can be equally frustrating to the visitor. You have only seconds to capture the attention of your website visitors, and a splash/intro page can cost you just that. In the time you should have been capturing their attention with an easy-to-use site, you've managed to push them to the "Close" button.
#4 Auto-Play (Audio and Video)
Never, ever automatically start audio or video content on your site - ever. Maybe your site visitors forgot to turn down their speakers after listening to something else, and now your site will blast them. Maybe (most likely) your visitors will not be expecting sound and will be startled and frustrated by the burst of sound. If they want to play it, they'll play it. If they don't, the rest of your site should accomplish whatever you hoped to accomplish with the video or audio content.
#5 Pop-up Windows and Boxes
"Subscribe to my totally awesome newsletter!! Really, it's super cool!" If you have to sell your newsletter, eBooks, or other "mega important" wares that hard, they may not be as great as you think. Your value should speak for itself through honest and straightforward content and a polished and professional-looking site. Build rapport with your visitors, and they will subscribe without having to be pestered to do so.
#6 Scrolling Text
This is just annoying. What else can I say? If you need to call attention to announcements (if they're that important), then have a designated and prominent space for them on your site, but visitors don't want to have to "catch" the message before they can read and process it, nor do they want to have to wait for it to come back around.
#7 Animated Graphics
Again, this is just annoying. Bouncy, trouncy, flouncy, pouncy is only fun, fun, fun, fun, fun for Tigger. No one wants to see little stick men jumping across your site, nor do they want to see a man dancing (Oh, man! That guy that comes up on the bank sites - irritating!). Don't do it - ever. Trust me on this, it's not cool.
#8 Blinking Items
Annoying. Why people insist on using these types of things is beyond my understanding. If it moves, if it flashes, if it makes noise, if it blinks, it probably should not go on your site without an optional "Play" button.
#9 Glamour Shots and Inappropriate Photos
Okay, let's say you're a real estate agent. You're helping people with what is likely to be their biggest financial purchase of their lives. I'm going to guess that they want to work with someone who is the ultimate professional, someone who is responsible and has it together. If your photo tells another story, you might want to rethink it. The same is true for all other professionals. Your photos should reflect your business and ideal clients.
#10 Stale Content
You have a site. Its goal is to secure new business for you. Take an active interest in it and keep it updated. If visitors see that your blog hasn't been updated for six months, they'll wonder if you're still in business - not exactly the message you hope to send.
#11 Drop-down Menus
While I have used drop-down menus for clients in the past, I do not recommend them. Browsers are fickle things, and stability of such features cannot be guaranteed across all of them. Plus, using drop-down menus can require a great deal of dexterity on the part of the user, which not all users have.
#12 Too Much Copy (Especially on the Home Page)
Do you really have to explain who you are, why visitors should work with you, and what you do in that much detail before you can expect a sale? If you do, you'd be better off breaking down the content into logical steps based on your sales cycle. Trust me on this one, they will not read all that content. In fact, I'd be willing to bet that not a single person will. That's valuable real estate. Use it wisely.
#13 Copying Someone Else
Okay, if someone searches for a phrase online, chances are any and all sites that have that exact phrase are going to come up, and chances are, the person who visits your site will also visit those other sites. If it appears that you copied the content, your credibility will tank, and the visitor will go elsewhere. DO NOT COPY CONTENT. Be unique. Be yourself. That's the only way to build rapport with your ideal clients and customers.
#14 Large Images and Graphics
This just adds to the load time for your site and can cost you visitors. I have stopped including images in my blog posts for this reason. I do not like waiting for pages to load, just for some (likely) vaguely related image that doesn't really sell the story anymore than the title already did.
#15 "Creative" Navigation
Cool is overrated, and here we are again. Nine times out of ten, when a client starts a sentence with, "Wouldn't it be cool to have...," I say that it's a bad idea. Navigation is no place to start trying to be cool. There are standards that web users are accustomed to finding in a website. Your site should not be the exception. If it is and if they can't figure it out REALLY quickly, they'll go to something they can find fast - the "Close" button.
#16 Excessive Advertising
It should not seem that the only reason you even have a site is to make money from Adsense. There is no credibility in that, and most times, visitors are looking for credible. There's enough junk out there. Be a real and honest source of information, and the few ads that you do have will be respected as valuable resources.
#17 PDFs
If you have an article, publish it as actual text on your site, whether that be in a blog entry or on an HTML page. PDFs are aggravating to use, slow as molasses, and not good for search engines. Only use them where absolutely necessary (example, client contracts).
#18 No Contact Info
You've gone to the trouble to create a website in hopes of visitors being able to find and contact you to do business with you, but you don't include the information for them to do so? Tons of sites fail to provide their contact information in an easily accessible location. This will cost you business.
#19 Cumbersome Contact Forms
Don't ask for anymore information than is absolutely necessary. You can always get any other information that you need down the road, after you have built a little more rapport with the person.
#20 Slow Load Times
Avoid all the "cool" features, the huge and animated graphics, tons of ads, and all the other time sucks, and your site will load in a reasonable amount of time. If visitors have to wait too long, they'll just go elsewhere.