The results from this large-scale study by Nielsen Norman Group are significant not so much for what usability issues they identified to be most common, but the fact that these are still the same basic problems that have been around as long as websites have:
The big news? None of the top issues today is new or surprising. Web design has come a long way. But these persistent problems remain. Modern design patterns and aesthetics change, but underlying user needs remain the same. Users still need to find information, be able to read it, and know what to click and where it leads.
Hence, the authors come to the simple conclusion that baseline issues keep occurring because baseline processes are lacking:
Many, if not all, of the mistakes listed above could be easily identified by including user research and usability testing in the website-development process and paying attention to the findings from that research.
...but also an important reminder that learning is a gradual process - doing research and testing on something new rarely invalidates findings from earlier phases:
When doing user research, hold on to old findings. As design preferences change 5 years from now, those old findings may keep you from making the same key usability-related mistakes a second, third, or fourth time.
UX consistency, I believe, is the most significant challenge for many organisations in terms of website usability.