Are there any aspects of PhD work that do not translate well to distance learning?

In another question about distance PhDs (Do any schools offer teaching or research assistantships via distance education?), I received several comments noting that PhDs on-line are of inferior quality to on-campus PhDs. If what is seen of PhD work from PHD Comics is accurate, PhD students seem to spend much time working independently. So, what aspect of a PhD cannot translate to the on-line format? What aspect of the process cannot be effectively conducted via E-mail or any of the other various forms of on-line communication?