Nothing quite underscores accessibility and usability knowledge like direct experience.
I'm just now creeping back onto the big-screen internet after spending the first six weeks of my daughter's life using only a smartphone to connect, with one free hand at most. Combine that with a slightly bumpy recovery from surgery and all the sleep deprivation you can expect from life with a newborn, and I've had plenty of very recent experience using the web while bleary, impatient, and on a device smaller than my hand. The highlights (and lowlights):
If someone in an emergency situation might need to contact you via your website, you need to have your main phone number and physical address (if applicable) in large type near the top of your homepage. Anything else is hostile and irresponsible.
Trimming content because you assume mobile users won't need it remains a terrible idea, as Karen McGrane's been telling us for years. Wikipedia, I'm looking at you.
Slow load times make me hate you. If I've been staring at my phone for 30 seconds while your site loads bushels of unnecessary files, not only am I going to back out of the site, I'm going to mentally put it on my Google results blacklist. Likewise, if you override my ability to pinch-zoom, use a mobilizer that makes me swipe instead of scrolling, or adds pagination, I will go out of my way to never use your site again.
If you sell things online and don't offer Amazon Payments or PayPal as an option, you're losing all the people using small screens who are never going to enter all their shipping and billing info in your tiny form fields with their thumbs.
This is miles away from a comprehensive list of mobile usability problems, but I noticed these again and again, often on the sites of organizations smart enough to know better. Mobile-only internet use is only expanding, and this group of users is much too large to ignore. And don't forget—if you're sufficiently unkind to a multi-device user stuck on a small screen, you may find they avoid you on the desktop as well.