The show has incredibly bad pacing, that is definitely its biggest problem. Other shows with large casts of characters can do an excellent job of balancing plot arcs and making mostly everyone feel important. I have my issues with Game of Thrones and admittedly it has an extra 15 minutes of episodic runtime, but in that show they're able to balance numerous plot arcs and make them all feel important and full of interesting characters. Sometimes there are duds but nowhere near as frequently as Walking Dead.
A big reason for the pacing issue is that it has a huge cast but doesn't know how to manage it. I love character development, but it usually IS filler on this show. Characters basically only get developed if they are about to die pointlessly without leaving an impact. In fairness to the current season, a number of deaths have had an actual impact on character development - Glen, Abraham, Richard. However this isn't the typical trend. Remember Beth dying? What impact did that actually have upon anyone? Her own sister hardly ever responded to it. She was so meaningless in this regard that the very next episode was a fakeout of them holding a funeral for her - psyche, it's Tyrese's funeral instead! But that's the problem, character development shouldn't be filler but on this show it often is because it tends to not lead anywhere. A good example here from another show to illustrate my point; Breaking Bad, Marie's kleptomania pretty much did absolutely nothing for the show and feels like a pointless diversion - some of the scenes revolving around it were entertaining but ultimately her getting that development does virtually nothing to the show. Would the show change significantly if you removed that element? If the answer is no, you probably created filler.
If a character gains depth and dies pointlessly and is never mentioned again, was there a reason to develop them? You don't get brownie points for fleshing out a character only to discard them immediately afterwards. A death of a developed character should feel like a loss, not "oh he's talking, guess he's done tonight... who even was he?". That's bad writing, character development is cheapened if it does nothing in relation to other characters (it's fine for core characters because they drive the story, bit players have to have a reason to develop).
This show can still be entertaining (I critiqued the C4 scene but hey, in a vacuum that was one of the best set pieces they've ever had), hell I often find the bad stuff entertaining too for its trainwreck qualities. But goddamn Walking Dead, learn how to have multiple characters already, you've had 7 seasons and you just get worse and worse at having a large cast. I'm fine with sticking with this show as a sort of pulpy guilty pleasure that I watch with friends or family, but I'd appreciate even some minor efforts at making it flow.