It's less of an issue these days, but filler that adds nothing to the overall story and it's characters is irritating for sure. I don't mind filler at all when it actually expands on the characters, world or story but when it's just padding it feels like I'm wasting time.
Also, unfinished series. Either because the anime is just treated like an extended ad for the manga/LN/game or because they are unable to secure a second season. I understand that it can be hard to sell your corporate overlords on a second season they don't think will be profitable. All I ask is that you endeavour to have seasons end on something that feels like a conclusion and not on an episode that feels like an ordinary episode.
i agree with gimmepie. filler is a killer especially for people who mainly have watched Shonen series like me. over time nostalgia builds, or people kind of convince me to like certain shows because they like them so much. like one piece which i didnt like for a long time because i thought it had too much filler. and i didn't like how it seemed that every person in the room needed to comment on everything that was currently happening in a kind of redundant way. watching the show now the stuff that i considered filler i actually don't mind so much, and enjoy sometimes so it all depends.
as far as the unfinished series which is also my second biggest, one of my favorite anime is magi. its the only reason i ever read any manga because they never finished the show. they built this huge world and then don't let u seee so many parts of it and they leave some pretty seriously unanswered questions at the "end" of the show about the over all story. ik that its probably because of what gimmepie said about the corporate overlords but idk it seems like some of these shows have such a short window to be able to survive. im sure i would understand better if i actually knew what they base their decisions off of besides like blu ray sales, which is crazy imo because who the hell is buying blu ray? all ik is i just want magi season 3
another one to add tho is these new shows that act like u have been familiar with the world for years on episode 2 if that makes any sense. similar to when a character will make it seem like another character is so stupid for not realizing their specific thought they had because it was so obvious but rly it kind of just 1 option of many possible options and its even necessarily correct or inspired at all. kind of a classic element of anime i guess and def has its value or its moments imo, but sometimes it just doesnt hit