Please stop doing the following.
Creating external links without a default target=_blank. You might be against it for whatever religious reason, but this doesn’t change the fact that people expect this behavior. I’m looking at you, Reddit.
Unnecessarily modifying the URL as you scroll through a page or highlight text. This makes me have to hit the back button 20 times.
Loading your title tags up with SEO garbage instead of my location on your page. This doesn’t help your SEO, and it makes tabbed browsing frustrating.
Infinite scrolling. Unless you can save the user’s position after they return to the page from clicking something, don’t do this. The user will have to scroll all the way from the top, wasting the user’s time and wasting your bandwidth even more.
Creating floating divs that pop up when you hover over text. I’m trying to read here.
Designing a website to be mobile-first without appropriately scaling up. This is just as bad as making a non-responsive website.
Needlessly using custom scrollbar solutions. These tend to break a lot and suddenly snap me back to the top of the page, or stop scrolling altogether. They also don’t scroll the way the user expects.
Using Hashbangs for AJAX crawling. That is so 2010. There are better ways to do this now.
Making black and white, animated on hover, circular cutouts for your About Us photos. Good lord, this is overplayed.
Outsourcing your blog to Tumblr. This is tacky. And what’s the point? Are you trying to anger the Google Gods? Or has Rails backed you into a corner with bad WordPress knockoffs?
Using slow CDNs. It sorta defeats the purpose.
Requiring users to use an app or sign in with Facebook. I leave every time, no exceptions.
Populating the meta keywords. No search engine since the 90s uses these. All you’re doing is giving the competition a glance into your marketing strategy.
Skeumorphism. No more cloth and wood backgrounds.
Java and Flash. The world really needs to move on now.