Series Good, but uneven
There is no doubt that there is a LOT to love about this series. It was generally well-written, and very consistently well-acted. The series finale is definitely one of the top ten finales of all time, IMO. However, two things made the series good, instead of great. First is the "main" character of Nate. I know it's an ensemble cast, but there's no doubt that his story is what defines the arc of the series. It begins with the start of his story and ends with the end of his story. Unfortunately, it's one of the weakest stories in the series. Nate nevers really changes as a character. He's as clueless as to who he is and what he wants at the end as he is in the beginning. The second factor is one that's a risk with any long-running ensemble-based drama series. Eventually, you start going "Really? How much crap can happen to one family? Really?" Much like with soap operas, you look at everything that happens to these people and starting to wonder how much the universe must hate them, because nothing good ever happens to them. You know that if it seems good, just wait. It will turn to crap. But if those two issues aren't deal breakers for you, this series does earn all of the accolades it gets.
Series Without a Doubt, the King of Complexity
"Six Feet Under" is not a perfect show, and that's mostly due to the many unfortunate choices of plot arcs that occurred in Seasons 3 and 4, which I think turned off a lot of viewers. The show, can, at times, border on melodrama, and, as a previous reviewer stated, it seems unlikely that so much would happen to the Fishers.
However, I'd still call this show one of the greatest television shows to ever air, if not THE greatest; because what it lacks in that opening paragraph, it more than makes up for in complexity.
Alan Ball may be the most talented character-creator ever. He created beautifully complex and flawed characters in "American Beauty." But here, the word "complex," referring to the characters, would be a criminal understatement. One could spend hours dissecting even a minor character like Vanessa Diaz and still not get to the bottom of it. Let alone the major characters. I don't think that anyone outside of the writers' room and the actors, maybe, will ever be able to fully dissect the most complex characters on the show; Claire, David, and Brenda. Emphasis on "Claire."
But of course, that's all in idea-stage - what about the execution? That's great as well. The show is, in my opinion, the best mix of comedy and drama in television, save perhaps "Bojack Horseman." This is an effective device for the quality of the show, too. Whenever they edge on melodrama, they catch themselves with humor. This is everywhere in the show, from the pilot episode, to a "double chubby," to "NARM," to the very last episode, in which, in a flash-forward, a certain character known for being boring literally bores another character to death. The show, in a sense, makes itself immune to its own faults. The writing is unparalleled, the acting, especially from Griffiths, Conroy, and Hall, is superb, and the stretch from "Ecotone" to "Everyone's Waiting"... well, suffice it to say it has one of the best ending stretches in television, and it certainly has the greatest finale ever.