One of the reasons for its sucess has to be because it has been starting off slow. Arrow, while featuring DC comics characters, making use of a major conspiracy, that only Oliver Queen can destroy. That kind of slow, arc-driven kind of story-telling has become quite popular over the years and it's good to see it used here.
Another great aspect of the show is the constant drumbeat of DC characters making a cameo in the show. Everyone from Deathstroke to recently Count Vertigo, it seems that the writers understand the history of Green Arrow and the DCU and want to make it apart of this TV series. To be sure, the characters are altered from their Comics origins, but at least they are a part of the newest continuity. The only complaint I have on this use is that in Arrow, they happen to be killed off by the end of the episode. Aside from the problem of having a superhero kill with such regularity, I just doesn't seem right that Arrow's rogue gallery is being so easily disposed of.
To sum up. Watch this show.
And as Always, Enjoy.