Well, it’s hard to put in words, but I know it when I see it. Like other good acting, it seems “real”, doesn’t scream “I’m acting”. Two good examples of TV acting off the top of my head: Hugh Laurie, of House. Julia Louis Dreyfus of Seinfeld. They stay in character, week after week. Their reactions to others’ lines seem natural, not forced. And, important for TV acting, you don’t mind watching them show after show, even if their character is unlikeable.
If we’re talking about what acting skills are most useful in the medium of television–as opposed to stage or screen acting–I think a few general statements can be made.
I’d say the ability to deliver long-term elements of character is essential, making him/her a believable or at least interesting person you’d want to watch week after week. These elements by-and-large are a result of the actor analyzing and understanding his/her character from episode to episode. It’s easy, for example, to play a shrewish wife once or twice, but for Patricia Heaton to play that character over an eight-year run requires her to find a lot of qualities beyond the cartoonish stereotype, or the role becomes stale.
This is why I’m not really a fan of the Emmy selections for Best Actor/Actress. Once you develop the character to the point that your work is recognized with an Emmy, you shouldn’t IMO receive future Emmy consideration for the same role unless the character changes in a substantial way. I mean, Tony Shaloub certainly deserved his Emmy for “Monk”, but once he got that character into a believable and admittedly fascinating state, what did he do to justify two more wins? There’s a difference between maintaining a character and building one up from scratch; the former is a lot of work, the latter is the fruit of a lot of work.