A filler episode, by definition, is one that is unrelated to the main plot arc. I'm not sure how you define a filler episode with a series like the original Star Trek where there was no wider plot arc. But then maybe saying filler episodes was the wrong thing? Deep Space Nine's crossover episode with "The Trouble with Tribbles" is a filler episode, but it's so good who the hell cares? So I would have to admit that being a filler episode isn't a bad thing. So I will withdraw my comment about filler episodes and try to better convey my thoughts without being as wordy as Snarf.
With longer seasons, very often the writers don't seem to have enough to put into a series and things just drag out. Season two of The Walking Dead has some great episodes, "Nebraska" being perhaps my favorite, but because they were obligated to produce 22+ episodes there were so many episodes where very little of consequence happened. And because they don't have enough story to stretch out into so many episodes, they fill it with pointless crap that ends up slowing things down. Did Lori really have to get in a car, go after Rick, and stupidly get into a car wreck? No. Was it the least bit entertaining? No. With tighter plot development we could probably cut 5 episodes from season 2 and not miss them at all.