I think part of that comes from CW shows (and a ot of genre shows in general) going with unknown actors without a lot of experience. If the show goes on long enough, they're able to grow as actors and fine tune their characters.I can give the CW shows a lot of shit for the acting but Arrow's cast deserves kudos for getting better with time. You haven't seen it yet but this past weeks episode was one actor's strongest performance yet, IMO. I don't think the same can be said for Flash unfortunately.
David Boreanaz from Angel S1 to S5 is another really good example of this.