It wasn't commercial. It wasn't conventional. It dealt with issues not often talked about in the early 70s, both the social issues that permiate through the series, and also such issues, in certain episodes, as prejudice, suicide, and homosexuality.
This is the story of the Bellamy household at 165 Eaton Place, London, both the upstairs family (the Bellamy family, led by Richard Bellamy, a member of Parliament) and the downstairs family (the servants, led by Angus Hudson, the butler, who in his way is more aristocratic than the aristocrats). Yet in many ways, they are a single family, and we see them from the period 1905 to the 1920s, an era of profound social change, and we see the effects such changes have on this household, from a time when going "into service" was routine to the time when having half a dozen servants for a small upper middle class family such as the Bellamys was beginning to be the exception, not the rule.
The series includes rarely shown episodes from the 1st season, as well as the special, Upstairs Downstairs Remembered: The 25th Anniversary Special. While the special is included with the first series episodes, I would advise waiting until you have viewed the entire series before watching the special, to avoid any plot points being given away.
The acting is wonderful, led by Gordon Jackson (as Hudson, the butler), David Langton (as Richard Bellamy), and Jean Marsh (as house parlormaid Rose Buck). Marsh also originated and guided the series. These three characters seem like rocks, upon which the waves of the social changes beat. Yet they are worn and changed by the events of this incredible era. Nonetheless, this is very much an ensemble cast--no character appears in more than 60 of the 68 episodes.
The first season seems almost experimental--many of the episodes have specific themes, such as those mentioned above. A couple were unsuccessful and their events are never referred to again (for example, "The Swedish Tiger"). In the remaining seasons, events tend to build over the thirteen episodes, to culminate to some extent in the final episode of the season, which usually deals with a major event in the world (for example the King's death at the end of the second season, the start of the war in the third, the end of the war in the fourth).
Perhaps the most powerful episodes are those dealing with World War I, and the profound waste of the war, as many of the best of the generation are lost. By the end of the War, there has been tremendous tragedy, and even the most ardent supporter of the war doubts the justice of the war. But do not underestimate the fifth season, as the social structure crumbles. The signs of this crumbling are seen throughout the earlier episodes, but they come to a head in the fifth season.
I have tried to avoid discussing the plot, so as not to give away the plot events that should come as a surprise to you. But suffice it to say that this is one of the first series when anything can happen within the framework of the series, when you could not count on everything ending happily by the end of the hour--or at all.