The disconnect between the end user and the Developers
When I think about the progress we have made specially in the last decade when it comes to Internet, it is almost scary. No me having background as an web developer can keep up with most of the new trends, but we do still have an aging user base. This user base is finding harder and harder to cope with the challenges of "being online".
I am more of a backend developer, but have moonlighted as an frontend from time to time. The most valuable lesson I learned as a frontend was how to create more intuitive UX for end users. That experience also reflects today in backend code.
Just because I try to be intuitive, doesn't mean I am always successful. For me it is very important to have a small group of people from my target audience to test my design. It does not make things fully 100%, but still gives you a lot of useful insights that you might have missed inside you little make-belief bubble of reality.