Something funny I noticed recently is that a lot of people, myself included, have dismissed seasons 3 and 4 of Arrow as outright bad, yet in most of the episode threads for those two seasons, the majority of comments are positive and ratings out of 10 are good. It seems them messing up key moments in the season, certain decisions taken in some episodes, and storylines and scenes in some episodes focusing too much on things that people do not like, such as Oliver and Felicity's relationship, is enough to ruin the whole season for a lot of people.
I can really only comment on Season 3, since I didn't watch much of 4, though I probably will later.
Season 3's issue was that it almost immediately utilized long, multi-episode arcs, and that those arcs largely suffered from one key flaw or another. Individual episodes could be good, but holistically they fell apart. Seasons 1 and 2 had flaws in their main stories as well, but they were both limited in number and in impact: the love triangle for Season 1 fell apart in execution, but was ended by Tommy's heroic death, and while Slade and Shado never really fell in love, Mirakuru had driven Slade insane, so his ramblings weren't nearly as impacted by that mistake.
Season 3 was beset by a combination of lazy writing, predictable plotting, wasted potential, and inconsistent characterization:
-The biggest issue with their long-term plan for the season was that they did almost nothing to disguise the fact they were ripping off a Batman vs Ra's Al Ghul plotline, and then tacked on a sorry ending for it as well. Seasons 1 and 2 had at least remixed their ideas: Malcolm is Tommy's father figure instead of Oliver's and succeeds in ruining the Glades, Slade is the antagonist of two timelines and has a viscerally personal history with Oliver. In contrast, Season 3 literally does a beat for beat copy of a Batman story, and ends with an unspectacular climax.
-Aside from the predictable nature of the main Ra's plotline, everyone on these boards saw Sara's death as a fridging for Laurel's benefit. It was blatant, unimaginative, and wasn't helped by how stupid it's reveal was in The Climb. And while Laurel improved during the Brick arc, we had to put up with her going through a boring revenge story and impersonating her dead sister bit. You could have had Black Canary without killing her wildly popular predecessor, and should have at least had a better plan for how to execute Sara.
- Merlyn was a mess. This guy has a triple digit body count of civilians, including his own son and Oliver's best friend and father. He's prime arch-nemesis material, and he seems to have become that eventually. But the iron was hot the second he and Oliver met, and they wasted him by having him both be defeatable for Oliver but somehow too dangerous to imprison (unlike, say,
Slade), trying to portray him as a loving father towards Thea while also being ruthlessly selfish enough to drug her into a personal weapon, clever enough to become the new Ra's but reliant on everyone making stupid decisions and only surviving because of luck anyway, and just generally a Villain Sue.
-They wasted the potential from The Climb in regards to Oliver himself and their season arc. It was decent enough for the other characters, but was pathetic by having Oliver just survive. You could have used the Lazarus Pit. Mirakuru, or any one of a dozen things, and instead he just needs a bandaid?!?
-They wasted Slade, Wildcat, and Katana. That's just sad.