In my work pitching series ideas and writing pilots (and on good days, selling them to networks such as NBC and Fox), I've learned a few things about what they're looking for, and what makes an idea sellable - as well as what a successful pilot script tends to include.
Most of it I've learned the hard way, from having people not want to move forward with one of my projects at some point in the process - be it the network, studio, producers, or even my own agents at CAA. When your project gets "passed on" at some point in the process (or professionally evaluated by someone like me), hopefully you get some valuable feedback about what might be missing.
Here is the number one lesson I've learned from that process:
Don't think of a series as one long story, but as 100+ little ones.
Most of us writers seem to be most interested in how a character and situation arcs over the course of many episodes and seasons, and draft that out in great detail in some sort of "bible." (Maybe it's because most of us started in features, where that arc is so central to the point of the movie.) When asked what happens in a typical episode, we might have a whole lot of possibilities of what could, in theory, happen. But what we haven't focused on enough, often, is the one thing that buyers and agents are definitely most focused on:
"What is the franchise?"
The "franchise" is the endlessly repeatable story engine - the problematic situation(s) for the main characters that is going to create a new story every week. Buyers and agents care far more about this than they do the season arcs. What they really want is a clear demonstration of what generates a story in an average episode - and how that story plays out in a compelling and entertaining way, which takes a full episode to resolve. They want to see that there is a seemingly limitless well of such stories, which are all variations on the same basic type of story problem(s) for your main character(s).
That's what they're going to be evaluating, and what needs to be front and center - focused on, and really working - to have a chance to get their approval.
A feature screenplay tends to be about a character (sometimes more than one) who has some sort of problematic life, then is hit with a catalytic event which leads them to develop a goal for that story. They then pursue the goal, and things get more complicated, difficult and urgent as they do so - until finally there is some resolution in the end.
The average television episode is basically the same. Whether you're talking about HOUSE, EVERYBODY LOVES RAYMOND, SEX AND THE CITY or THE SOPRANOS (or any series), they all tend to begin with a problem or crisis that the series regular you're focusing on feels they must solve, to get things back to their normal status quo (however compromised and unsatisfying that status quo might be). It's very important to them, and they're basically in hell trying to solve it - but it's really entertaining for us to watch.
Buyers and agents want to know what's generating those problems every week, and preferably, it will be in the context of an idea which represents a fresh twist on a familiar genre that has worked before in television. (These genres are more specific and limited than what we might normally think - just as they are in features - but that's for another post.) Suffice it to say that the thing we must be most crystal clear on, in our pitch and our pilot script - is where these stories are coming from.
Although some series get by with the challenges of a heroic occupation or undertaking with huge, relatable, life-and-death stakes and an entertaining process to watch (i.e. certain kinds of doctoring, lawyering, police investigations or "adventuring") most dramas and virtually all comedies are mostly about what is loosely (and not pejoratively) called "soap" - the challenges of personal relationship conflicts for a web of characters, each of whom want something that is important to them, that they can never quite get.
In features, the main character tends to heal and grow and fix their life in the end. Not so in series! (Except maybe in the series finale.) Television characters tend to be pretty much the same from episode to episode, with the same unfulfilled desires - despite the incremental changes of circumstances that happen over time. It's the pursuit of those unreachable goals (and the problems that come up in life related to them) that are the stuff of story.
So the goal becomes demonstrating who these people and problems/goals are, and how a typical episode takes a full hour (or half hour) for them to resolve a big, specific problem related to them.
This is one reason why pilots should primarily be examples of a typical episode - and not merely the first installment of a long story, which only shows how everybody came together into the current situation. Their primary function is to represent what future episodes will look like, and thus sell the franchise to buyers.
If you can make your primary focus finding and then demonstrating such a franchise, you'll be on the right track.