Wikipedia, FTW.
Apparently it's only missing a single episode as well (1219), although it seems it was reconstructed with a home audio soundtrack and other footage/stills in the compiled release.
I am something of a DS scholar <adjusts monocle and shuffles stack of parchment>, having actually seen every episode at least once.
There was certainly no unified storyline. The series is best known as the story of vampire Barnabus Collins, but (as you point out yourself in your facts) he did not come onto the series until after episode 200. Dan Curtis' original idea was to make a "gothic" rather than "supernatural" soap. It was to show a once mighty but now declining (perhaps cursed) family, the Collins. seen through the eyes of a new governess, Victoria Winters.
Originally every episode was introed by Vicky doing a voice over--this was eventually replaced by a voice over by any actor otherwise appearing in the episode as Dan discovered he would have to pay the actress playing Victoria whether she did anything else in the episode or not (and they were on a shoestring budget).
The show was not initially successful--the first major arc included Vicki's pupil, David Collins, trying to murder his father by cutting the brakes on his car--and the subsequent coverup of the whole thing by the family. The next major arc was the arrival of David's estranged mother who turned out to be the first pseudo-supernatural creature, a phoenix. Actually David did have a friend who was a ghost prior to this, but for a long time the show played coy that this ghost friend might only exist in David's imagination.
When Barnabus finally did arrive the show finally took off--and completely embraced its supernatural bent. Several months later the show took a drastic step and actually shifted all the action back in time...and stayed there for a very long period, setting the paradigm of alternate periods, and alternate universes that it utilized until the very end (the series actually ends in an alternate universe).
That's just my primer...
Anyway, like any soap, there are no "stand-alone" episodes, altho you can easily jump into the series at any point and get up to speed pretty easily. The two big villains, Barnabus and Angelique, eventually make 'face turns' becoming heroes, which could be confusing if you skip a bunch of the middle. And my personal favorites are the episodes featuring Quentin, the family werewolf.
There were two movies made using the cast of the soap. The first,
House of Dark Shadows, condenses the Barnabus storyline into a two hour feature. It is really underrated imo, but certainly not art. The second,
Night of Dark Shadows, is a reimagining of
Rebecca and any number of witch-hunting movies. It was actually completed just after the series ended and stars Kate Jackson who got her start on the soap. More importantly, both movies feature the fantastic (and fantasically camp) Grayson Hall, as well as Nancy Barrett (the only actress to be in the soap from first to last--with one short break--and both movies).
