Static images and graphics are fine, and user-triggered movement is fine, but the all-dancing, all-singing, in-your-face sites can please take a flying leap at a rolling doughnut.
TBH, I even find distracting text annoying - Stack Overflow's "Hot Network Questions" is a good example. I'm looking at the details of how to parse version N of format X, and it is not at all helpful when my eye catches: "If a dwarf barbarian wears heavy armor how fast do they move?" (actual HNQ question, no offense intended to the querent, dwarves, barbarians, etc.)
Banking sites that still use verification questions
Landing pages that don't take you to the actual content (such as if you type drive.google.com into the url bar when you're logged in to Google and have been going to the same site for a while)
Lots of Tracking
- Basic functionality broken when JavaScript is disabled
- Unnecessary use of iFrames and other subdocument elements
- Loading megabytes of libraries and scripts for a basic menu animation that could be done with CSS3 or plain JS
- Taking me off-domain to log in - the logged in area should be dashboard.example.tld, not exampledashboard.tld (mainly banking sites do this)
- Requiring logging in after visiting an email address verification link
- Sending email verification links, I would really prefer verification codes that don't require visiting a link
- HTTPS mixed content errors
- Messy HTTP response headers, such as invalid values, duplicate headers, or default values for everything
- No easy way to find a contact method for the security team, such as the RFC2142 security@ email address, or security.txt
- Videos autoplaying on some news sites
- Cookie prompts
- And the worst: overlays that appear on scrolling
Having to dismiss something is an utter pain.
Missing functions in the mobile site.
Too much white space. (Ie new reddit)
Popover prompts (this catches a lot), autoplay, scroll-jacking, ads, cookie notification, geo-blocked, paywalled, pop-ups, middle-click for open new tap does something else or does not work because it's no real link
- Asking for registration/login, just to get spammed later with annoying newsletter bullshit.
- Everything else others said already.
2. Reddit install app nag
3. Paywalls on media/news sites