It has nothing to do with CW. It has everything to do with the writers dropping the ball.
Season 1 started out as your typical CW show, but the writing really improved in the second half, give or take a few parts here or there. It's not exactly great or a masterpiece or anything, but it's good. Then the ending didn't make much sense in the context of the season (specifically when it came to "the list" and what it is), but I forgave it because it was a good ending out of context. It was clearly one of those "the buildup got more and more complex/better than what we planned at the beginning" sort of ending, so I didn't mind. Overall the the season ended strong, and there was a feeling of comfort going into season 2 knowing the show found its legs and the writers knew what worked, what didn't worked, and how to better balance out a year-long story next time.
Then Season 2 starts, and the first part is absolutely incredible (minus everything with Laurel). There's a lot happening, but the writers do a good job of balancing all the subplots. Then in the second half, everything falls apart. Characters conveniently get written out for several episodes in a row only to come back later and feel forced due to a lack of proper development. Biggest examples are Sara and Isabel. Certain subplots don't come together that well, mainly related to Deathstroke. Deathstroke's motivations are also poor and forced, with the writers expecting us to buy everything he does just cuz he's insane. Not to mention his whole plan is full of contrivances and, to a certain degree, requires some omnipotence on Slade's part. Blood also feels superfluous to Slade's entire plan, and also out of character in the season finale. Everything slowly starts going downhill after episode 14 or 15.