For those who follow this area, it should come as no surprise that Polchinski does not provide a complete answer here. As physicist Clifford Johnson points out on the Asymptotia blog, no one - not even an expert like Polchinski - is 100% certain what string theory actually is.
This is not, as some critics claim, necessarily an inherent failure in the theory. Keep in mind that even after about a century of work, there are still several interpretations of quantum theory, so the fact that thirty years doesn't nail down all the particulars of string theory is hardly surprising.
As Johnson points out frequently throughout his blog, the major failure of string theory was in suppressing unrealistic hype since the mid-1980s. Physicists began to make bold statements about the theory, goaded on by journalists aching for exciting headlines, and the result was a theory which was almost destined to falling short of expectations.
Still, if you're interested in seeing one of the experts in string theory present the current status of the theory, and look into the key elements that need further work, this is an excellent presentation. (The video is jumpy on the download that I got, but the audio is great.)