As good as Dark is, he does tend to dominate a bit, McDonough being a better actor than pretty much anyone else on the show except for John Barrowman, with the scenes between the two being some of the best in Arrow's history. Dark is also introduced a bit early, hurting the idea that he's the head of a massive organisation, what with his having to get his hands dirty all the time. There's also a buildup to a death of a member of "Team Arrow" that is foreshadowed far too long, almost as if the producers and showrunners were afraid that people wouldn't keep watching the programme without some kind of continuous "teaser" to bring them back each week. With a bit more faith in the material and a producer willing to take a small risk here and there, Season 4 of Arrow could have been the height of the programme before it really started to drag in Season 5.
I haven't really touched on Oliver's run for Mayor or the Legends of Tomorrow crossover, but they play as filler more than anything, with no real resolution for either; yes, Oliver becomes Mayor, but only because of the death of his opponent. Oliver's relationship with Felicity becomes more of an irritation than anything, with her dumping Oliver every few episodes because he lies to her. Even ranting about honesty and betrayal when Oliver has only obfuscated the truth temporarily to save lives. Anarky is a recurring character, with a pretty cool look, actually, and more than the typical "bad guy" motivation, pity he's killed off and won't be returning. Season 4 is worth a look, sticking out until the final half-dozen episodes does pay off, even if it's not perfect. This marks the end of "Team Arrow" and would have been a good place to end the saga of Oliver Queen, but looks like CW want to milk this turnip a little more yet.