I don't doubt that all these are repeated elsewhere, in 1000's of pages. That's the good, and bad thing, of the Internet. There's a bunch of information. Unfortunately, especially when it comes to IT, much of it is out of date (or not updated), totally wrong, or written by armchair amateurs. You know, some of the same ones that clog up the Internet with horrible web pages that start with "Welcome to my page" or similar.
So let's get started......(in no particular order)
1. Bloated files/pages.
This could really be 2 categories. Firstly bloated pages. These are the ones stuffed with gigantic pics that allow you to cook dinner, wash the car AND do you shopping while the page loads. Second are bloated files. PDF's are great used properly, but they can be big. I like to offer my visitors the option of saving/opening a PDF, or viewing as a plain HTML page. Yes it takes more work on the part of the designer, but it's much more user friendly.
2. Visited links that don't change color.
When users have visited somewhere, they like to know it. Not coding links to change color when they've been clicked is insulting to your visitors.
3. Spelling and grammatical errors.
As designers, we're trying to impress people with our writing and design skills. And every program seems to have a spell checker tucked away somewhere. Use it! Grammatical errors aren't usually caught by spell checkers, so I always try to have at least one, better still 2 or more, people proof everything before it goes live. Words are one of our stock tools - using our tools badly doesn't inspire confidence!
4. Splash/landing pages.
They used to be all the rage, and many designers and SEO people abused them, using them as bad doorways to worse sites. There's no purpose in a splash page. Optimize the homepage and use that instead. I get really frustrated landing on a page that just has a "cute" flash intro, even if "Skip intro" is somewhere visible (usually in 2pt font!).
5. Auto start music.
Possibly my pet hate. Even when designing music sites I wouldn't assume every visitor wants to be blasted with music just because they visited. A music site is almost forgivable - any other site has no excuse. For goodness sake, if you feel you must have sound, make it user manageable!
On the topic of sound, why do so many designers think that they need to have a little click sound whenever a link is clicked? If the navigation system is good, and rollovers are active in the menus, people will no when they've clicked!
6. Best viewed in
To translate, "I was too lazy/too incompetent/too stupid to test my site in any of the other browsers, so I'd rather you wasted your time changing browsers to ...!" The audacity. There is no excuse for not testing on every browser to ensure compatibility. The same applies to "Best viewed in resolution".
7. Welcome to my page/my home page/XYZ Company, etc
Yawn. Who cares? And it'll do you know favors if you're trying to get sensible text displayed if you're lucky enough to have someone find you in a search.
Counters are easy to put in, and can't be relied upon to tell the truth for a number of reasons. On eof the main ones is the site owner! He/she won't want a "0" in there so will invariably start at a number higher that "0". And it's just amateurish.
Come on, you want a professional website right? Sure, have a testimonials page that clients can post to, but not a guestbook.
We're the best, the fastest, the cheapest, the leaders in our field. Yeah, so what? If you don't answer the following - WIIFM (what's in it for me?) your visitor will leave. You need to offer solutions, not 30 pages of self-back slapping.
11. Non-professional graphics
Does the site look like an elementary school kid made them? That tells visitors something about you, and it's not good!
12. After 5 seconds you still don't know what the site's about
If a new user looks at your site, will they be able to tell what you're all about after 5 seconds or less? If yes, that's a good start (as long as bad auto-start music doesn't drive them away). If no, then you have serious redesigning to do.
13. Where on earth am I/where do I go/where is the information I need?
Or, put a different way, your site has bad navigation/menus. Not only will this seriously hamstring and annoy your visitors, search engines will have a really hard job spidering your site.
14. Disabling right-click
Yes, it seems a great way to protect your oh-so-valuable photos, etc on your site. The truth is, it's easy to circumvent, and seriously annoys users who use right click for other than taking images.
15. Non-standard/conventional layouts
I see it all the time with advertisement or affiliate type sites - a single page that's a mile long. I, personally, can't be bothered scrolling for an hour. And I'm equally unimpressed when I click a link and get taken to a page where the top left menu is now at bottom center.
16. Forcing unnecessary registration
This really infuriates me. Why do I want to give my private information away unless absolutely necessary. Sure, if I'm buying or registering for a something, then of course. But if you're offering a FREE download or similar, don't demand to know everything about the visitor.
This is by no means exhaustive, nor will everyone agree with every one. There are some pet peeves in there (though most are well shared with other Internet surfers. Not exhaustive but a start! Happy surfing!