For example, state preservation is typically never implemented completely. I don't think I have ever worked with ONE web application that nailed this 110%. There is always one page in a wizard that when you click back is cleared of it's previous contents, etc.
One thing that really annoys me is typical record listings with advanced filtering options. I find the record I want viewing browsing or searching or a combo of both, begin to edit details, save or cancel and BAM the record is updated but the filter state is lost and I have to find my record AGAIN in order to validate whether the save was successful.
I've seen some apps which try and circumvent this issue by using extensive AJAX. Problem is, when Javascript is disabled, nothing works. Even Facebook doesn't seem to work 100% when JS is disabled.
What do you often find missing in web apps which make you uneasy about using it, because as a developer you know it could be better?
Cheers