In the comments to this post, both Mac and Isaac bring up an intriguing point: when people say that something was like television (or a movie) on stage, they mean it doesn't take advantage of the unique qualities of theatre. (Chime in if I'm mischaracterizing your point, fellas.) That puts me in mind of this discussion. That's definitely another way to look at it: what qualities does theatre have that are unique to live performance? And how does that separate it from other forms of dramatic writing? Once we head down that road, I think things start to make more sense. One of the moments mentioned in that discussion is the moment in the second act of August: Osage County when all of the characters, and there are ten of them on stage at that time, start talking (and moving) all at once, as they settle down to dinner. It's a startling moment and quite "theatrical." That kind of thing really doesn't work on film (Altman aside) or on television. Part of the power of it is that it is actually happening, just like that, right before your eyes.
Most, if not all things that happen on stage, are not natural. Same as on television or in film. I think we're so used to thinking live=natural and it's just plain not. That's why so rarely do you see the most common tricks of the theatre trade work on film: double-casting, the unity of time, place and action. Because we're used to naturalism on film and that's not what we get on stage. I've mentioned it before, but the closest I've ever seen to naturalism on stage was Adam Bock's The Thugs. And that made it down right experimental. We exist in a realist art form, the representation of life, not an exact reproduction. Mamet famously said that if you take all of the repititions and false starts and banality out of regular conversation, you have dialogue. But we want it to feel real, to feel natural. It's a tricky balance. And, in general, it doesn't really work in other mediums. Almost every time a play gets turned into a movie, the usual knock is that it's too "stage-y." People, especially in movies, don't sit and talk to each other the way we do on stage. It's an advantage I think we forget about, in pursuit of some moment of stagecraft or whimsy, or revolutionary storytelling. People just talking can be theatrical. There, I said it.
And storytelling that depends on people talking IS theatrical. When you write a film script or a television script, you have to think differently. In the same way that if you were to try to write a novel, you'd have to conceive your story differently. We tell stories that (largely) depend on people talking, communicating at least, in one form or another with each other. You can't write a play that consists only of the internal thoughts of someone, as he sits drinking tea and eating a madeline. But you can make a novel out of it. All right, yeah, okay, you can, but you're moving into performance art or dance-theatre. Not necessarily playwriting. So, when you sit down to write a play (or when I sit down to write a play), I try to think of a story that can be told in this medium, in people talking, even if they're talking about something other than what's going on. A story that unfolds in action and dialogue and images in a certain way. If I were going to tell the same basic story in a film, the needs of the medium are different.
I don't necessarily think every single play works as a play, or can't work in another medium. But sometimes I think we dismiss the realist nature of our work, to our peril.