Jakob has posted his Top Ten Web Design Mistakes of 2003. The good news is that sites are getting better at using minimalist design, maintaining archives, and offering services. Which leads right into the problems.
Now, let's be honest. There will ALWAYS be problems. You can't be everything for everybody. But it's good to try.
And also of note, these mistakes aim at corporate/professional/commercial sites. I don't count personal sites (such as this one) in this list. Personal sites are meant to be exactly that, personal. Which means what I want. Maybe I don't care about my users (I do though). Maybe I just want a place to experiment, or play, or whatever.
I'll summarize his listing here.
# Unclear Statement of Purpose
# New URLs for Archived Content (I'm a big believer of this one. Linkrot sucks)
# Undated Content
# Small Thumbnail Images of Big, Detailed Photos
# Overly detailed ALT Text
# No "What-If" Support (I feel this is overly optimistic. I wouldn't call it a mistake, rather than a "I wish")
# Products Sorted Only by Brand (again, the assumption is that every site out there is dynamic and has a dedicated team of programmers. I'd put this in a "I wish" list)
# Overly Restrictive Form Entry (another "I wish" item)
# Pages That Link to Themselves (I really don't agree with this one. I feel mucking around with the navigation based on where you are is ridiculous, but I could be taking it wrong)