I think traditional relational databases (RDBMS) are generally crappy for as the data store for complex dynamic web sites. But despite the inflammatory nature of that first sentence, I'm going to try not to turn this into a rant. Instead, I'll try to explain the problems I see and why I came to that conclusion.
The problem with RDBMS
First of all, I don't have a problem with RDBMS. I think it has served us very well up to this point and will continue to do so for the forseeable future. It's a perfectly workable solution to the problem of storing and retrieving persistent data.