|Think about Loose Coupling|
I see responses mention content suppliers as well as programmers and designers, but I think you're still missing an important element, the 'user interface architects' (for wont of a better word).
Essentially, someone who can take all the elements of a site and ensure they fit into a useable framework. No more mystery meat navigation. Please! In fact, if you are working on web design/programming, I think the whole of Vincent Flander's site is a must read. (and no, I don't do my company's web page :)
Another, is having a web site that doesn't supply what the user expects to find there. My pet hate on this front is my bank in the UK for one, very small, reason. Last year, I was on vacation and I lost my bank card. I wanted the bank's 24hr emergency phone number to cancel my card. Was it on their web site? Errr... (and I still can't e-mail my bank manager ffs!). In fairness, I'd guess that most traditional banks don't know their asses from their elbows when it comes to web presence, but why not? Two or three years ago they could be excused for this (just). But now? Tch.
When I did web design, I had an absolutely fantastic designer, but he'd start having fits whenever I lectured on web safe pallettes. At the other end, I've worked with a certain financial institution that had a really powerful - but awkwardly designed - cc transaction system.
Gosh, I'm rambling. Sorry. To sum up though, I think balance is needed in these four corners:
Doesn't have to be 4 people, just as long as the 4 skill sets are covered and they communicate with each other to ensure they build something cohesive.