It can be said without fear of contradiction that, a far as DVD releases go, Doctor Who is the most extensively covered series of all time. If you consider that every story of the classic series gets at least one documentary and other featurettes, while the new series is extensively covered via Confidential, video diaries and commentaries (even if they don't do every single one). If only other DVD TV series releases were as in-depth as this.
It must people "outsiders" who have no knowledge of Doctor Who to see how heavy the coverage is. Can you imagine NCIS or 24 having a weekly one-hour documentary series devoted to every single episode? Doctor Who has (and some Confidentials have rivalled the main series for ratings, so people are watching them). Actually two, since we forget about Totally Doctor Who which ran for a couple of years as well.
Alex