This is an incredible show. Incredible. There is pretty much nothing more to say! *Most* of the acting is stellar, the storyline is interesting, and Julian Fellowes' writing and directing is some of the best. There is just nothing like watching Season 1 through Season 6, as you watch the changes that occur for every character and the Abbey as a whole. There is so much to be seen here: the slyness of the characters, the glances, the hidden frustration with each other that bursts out at some point...you could watch this show repeatedly and still not catch all of the undertones.
Let's start with the acting, and there are many characters to get through. Hugh Bonneville and Maggie Smith, the two best-known actors in the cast, are incredible and live up to their storied names. Smith's little jokes (most of which no one but she finds funny) are such a perfect comedic relief among the drama and grief that can sometimes drag you down as you watch. Laura Carmichael (Edith), Allen Leech (Tom), and Dan Stevens (Matthew) are incredible! They play their roles so well and really bring the characters and all of his/her intricacies to life. (I am trying very hard not to bring up Dan Stevens's selfishness in leaving the role of Matthew for "greater" things--not cool!) All of the downstairs characters are incredible--Jim Carter as Mr. Carson, Phyllis Logan as Mrs. Hughes, and everyone else. Kevin Doyle as Molesely plays his comedic role so perfectly, and he is legitimately funny.
There are a couple of low spots in the acting, specifically Michelle Dockery as Mary and especially Elizabeth McGovern as Cora Crawley. Dockery sometimes overacts and sometimes underacts, but overall I think her character was well-played. McGovern, on the other hand, is simply not well-cast. She overacts to the extreme, and her "dramatic" lines are just bad.
The set is (obviously) incredible, considering it is a real English castle. The plot is exciting and, as mentioned earlier, there is so much that you miss every time you watch it that you will always discover more of those little undertones whenever you rewatch.
Just an incredible television series. It is one of my favorite shows ever (in fact, it may be my favorite show ever), and the little acting inconsistencies do nothing to bring the show down in the long run. If you stick with it and watch the entire thing, you might just cry at the end of the series, as the downstairs staff sings Auld Lang Syne while the camera slowly pans away from the snowy Downton Abbey castle. The kind of nostalgia this show brings is unbeatable. Watch it. Just watch it. You won't be disappointed.