Aesthetically challenged
What's actually been the biggest problem is the design and usability - because I suck at that and I suck at DHTML and Javascript and CSS and AJAX and for all the pissing and moaning you get about Open Source stuff looking horrible there's no coherent resource for getting usability and designery type people to work on your open source stuff.
To be honest, I don't know who's to blame, per se - the programmers for being usability hostile or the designers for not getting involved. Chicken and Egg problem I suspect.