In honor of the anniversary, a few days ago I decided to watch the episodes that aired on June 27 of each year that DS ran. (In the case of 1970, I watched the episode that aired June 29 because June 27 fell on a Saturday that year.) I was curious to see how much the show changed in one-year intervals.
What I found was that the 1966 and 1967 episodes were very much of the same tone and continuity (with the latter referencing events in the former), but that the '68, '69, and '70 episodes had very little, if any, connection to each other. Each one was set in a different time period, and featured different cast members.
Even more interesting, Jonathan Frid appeared in only one episode, and Barnabas was rarely mentioned in the others. (Granted, his character didn't appear until 1967.) On the other hand, Alexandra Moltke appeared in three of the episodes, while Joan Bennett, Nancy Barrett, Louis Edmonds, and Humbert Allen Astredo each appeared in two episodes.
Because I've always watched the episodes sequentially, they've always flowed together and I could see the overall continuity. Watching them in this way really emphasized how complex DS was (and is), and how much was packed into those five years.