Aristotle's Poetics defines the essential ingredients of drama as:
Plot, Character, Theme, Speech, Melody and Spectacle
Greater minds than mine have debated and discussed these points and what they meant in the time of Aristotle and what they might mean in terms of today's drama at length, and will certainly continue to do so as long as there is drama for entertainment. I'm only going to use them as a guideline for what I look for in a story, any story, and here specifically in television shows.
Most of the shows below are character driven. House certainly, Bones and Medium are a small cast of characters, L&O is more plot driven, although in CI the resolution certainly hinges on the characters of Goren and Logan in their respective episodes, CSI can be a mix, although I watched it primarily for Grissom and maybe Warrick, but also for the plot. Since Warrick died and Grissom is leaving I wonder if it will still hold the same appeal as I never managed to be interested in either CI: Miami or NY because I didn't like the characters. Life is entirely and only interesting (at this point--it's still very new) for Damian Lewis' performance. Eli Stone was a mixed bag. 24 is actually mainly plot and has long stretches without Jack Bauer.
House, Bones and Medium all have great dialog as well. Really great dialog, and this, for me makes up in part for plots which are often not particularly exciting and rather formulaic. House is particular has a very definite shape to the hour, to the point where one can almost say, "Oh, the patient is suffering a mysterious reversal after seeming to get better? Must be 9:23. I'm going to the kitchen, do you want anything?" or "It's 9:47 and House is talking to Wilson. Here comes the seemingly unrelated thing that will give him his epiphany." It is also almost as easy in Bones and Medium to pick the culprit as in an Agatha Christie. Not quite as easy as knowing the red shirt is going to die, but close. Occasionally they stump me, and me more than my husband and the season finale was so much of a dark horse that I felt a little betrayed as if pieces were lost in the writer's strike that might have made the ending more believable--more on Dr. Addy's experiences in Iraq perhaps? I used to watch Monk with the quite brilliant Tony Shalhoub as the detective with OCD, but the ONLY reason to watch the show became his performance. The mysteries were laughable and the dialogue repetitive.
What they all have is great themes (and here I think I am varying widely from Aristotle's definition). That is the concept of the show--the maddeningly brilliant Dr. who is horribly damaged; the maddeningly brilliant forensic specialist who is quirky; the soccer Mom with visions, etc. What might be termed the "conceit" of the piece in poetry through which the deeper meaning might be seen--if we buy that television shows have a deeper meaning.
Speech we've covered--great dialogue can cover a multitude of sins for me. Melody--hmm, shall we take melody to mean the timing of the show? Then House is weak. There is a three act structure to most hour long televisions shows. It's considered TV writing 101. Set up of problem, deepening of problem, solution. Most shows follow it to a greater or lesser degree. Off the top of my head I can't really think of a show that truly breaks that shape. So we'll leave that out for now.
And the final piece is Spectacle--this has always been a hotly debated point about what it means in contemporary terms. Is it all spectacle since we can do so much now--greater and greater imitation of life or larger than life. Is that enough? Then Pixar is the master of spectacle, but is spectacle more than just an "Oh, Wow" kind of effect. Television and movies are always going to be more realistic than stage, but stage can be much more spectacular.
Let us use Spectacle here to refer to the bits that transcend the genre. Joss Whedon for instance is a master of spectacle. Buffy had an all silent episode and an all singing episode. Moments when Hugh Laurie is allowed to play instruments has something of spectacle about it (for me at any rate), of knowing that Hugh Laurie IS this accomplished musician--a meta moment, if you will. Eli Stone is primarily spectacle over everything else. The eponymous character gets visions in the form of musical interludes--primarily from George Michael, but with other bits thrown in.
I would say, for me there has to be at least three of the six present to make a show worthwhile. For House it's Character, Speech and Theme. For Eli Stone, Theme, Spectacle and Character.
I run into trouble when discussing Lost. I think the only thing that Lost has going for it is Theme, it's conceit of the plane crash and mysterious island. I find the characters dull and one level and the dialogue wooden. Certainly the spectacle can be interesting, but they've been traversing this island for awhile now--there isn't really that much to see and the "Easter Eggs" require frame by frame viewing which I am not interested in pursuing. If Lost were a book I'd have skipped ahead already. I'm watching only to find out the answers and every Thursday this season I'd think--is this really worth it? It isn't for my husband and often I find myself doing other things while it plays in the background.
A good friend has recently started dating a Lost fan. They've been watching his DVD's of the first season together. She (a seasoned theater veteran with an MA in directing) can often guess the next point or certainly, the next line. This puzzles him no end, but really the dialogue is that simplistic, the characters that repetitive. Enough, we get it. I have a terrible feeling that by the time the answers start coming I won't care anymore.
Having a set timeline can save a show because it requires the writers to meet certain points. There was a shape to Avatar that kept it tight and lean but with plenty of room for side tangents. Babylon 5 in the mid-90's had a 5 year shape--ambitious for a writer/director with few other credits to presume that his show would last 5 years, but that was what the story needed. Unfortunately the clever TV execs made him do it in 4 and then gave him a 5th year which had nothing of note in it. Well, d'uh. Contrast that with the X-Files. The success of the X-Files kept propelling it on, so that answers could never really be given and as a result the last seasons are feeble floggings of dead horses. (Can he possibly have resurrected it for this new movie?)
If you know you have a timeline, then you can progress, from A to B, B to C etc. You don't have to keep coming back to A so that you don't run out of story before your popularity wanes.
Actually, as I think about Melody, the further along a show is, the more it can play with it's own rhythms. CSI could have a season finale about a member being buried, or a background story of the miniature killer because so much has already been established. They can go from A to E in a story and need only return to C in the next episode. The Simpsons too can play more freely with the shape of a story--stories change direction all the time from what it seemed it was going to be about (sometimes successfully, others less so). The Simpsons (and indeed most cartoons) can go from A-Z in an episode and return to A at the beginning of the next without the blink of an eye. Homer has been famous too many times to count. Lisa has tried every sport out there. And each week Homer is back to his job at the plant and Lisa is back to being unpopular and Maggie never speaks. And we don't care because it's a cartoon.
Obviously in cartoons I require spectacle above much else, but we are very picky about dialogue and plot as well. Good voices (funny/quirky) help too. People assume we like all cartoons, but we search long and hard for a cartoon that can hold our interest--that strikes the right blend.
And, as I said I look for these elements in everything. There are writers of speculative fiction (a better term for sci-fi/fantasy, much of which is neither sci-fi or fantasy) whose concepts are amazing but unfortunately fail in terms of character and dialogue.