Imma use Steven Universe as my main example, because it's by far the most famous offender. ^^ It's hard to believe how long the show has been around, considering where the plot is right now...
Anyway, the thing you hear the loudest complaints about is how we're often forced to wait half a year for a week of filler, and maybe a special full-length episode at the end if we're lucky. Not gonna lie, I think it's a pretty rotten way to run a show, on several levels. =/
People often defend SU by saying the filler isn't a real problem, or the scheduling isn't a real problem (citing anime as an example), and I agree...it's both at once that's the problem.
You may see 13 episodes of an anime every year, but those 13 episodes are a fully-fledged narrative from start to finish (at least if you're watching a good anime), not just one fluffy piece of a decades-long puzzle.
And I think SU would have lost fewer fans if it operated that way: start things and finish things and THEN take your 6-month break. People will always be willing to wait IF they know it'll pay off. It's when they expect to be jilted and cheated that they start losing patience.
I tend to think about pacing as building and maintaining story momentum, always making the viewer feel that something has been accomplished, and then giving them something to look forward to. And this is especially important when it comes to TV and film, when the wait-time between story bits has always been longer.
And most of the industry seems to understand this (see 'cliffhanger'), so I don't know why some people think SU deserves to be a special case. Is it really that hard to believe that a clump of Beach City episodes does not contain enough story momentum to sustain interest into the next year? Or that an episode spent arguing a SINGLE character point during a CLIMAX does not make the viewer feel like something is being accomplished?
Anyway, that's enough Universe-bashing. Believe it or not, I don't hate the show, and I STILL think there's hope for it yet, even if the odds of it becoming a legend/classic are slim to none at this point.
I just think that, in the era of binge-watching, pacing a TV show correctly is more important than ever, because if you screw up you don't get another crack at it for a while. It's more than just scheduling well or moving the plot as fast as possible. It's understanding what kind of show you're making, what's actually important to its plot, what the viewers will expect, and balancing those three factors.