Sorry for joining this discussion so late - I only just got round to watch this show. I really enjoyed it, and agree that both Vickie McClure and Keely Hawes put in excellent performances, but I disagree with some of the criticism of Martin Compston's role.
Line of Duty managed to combine the grippingness and watchability of US TV with the complex characterization of UK TV. The problem with UK shows is that they forget they are primarily a form of entertainment (think Peeky Blinders), while in U.S shows, even if the characters are morally ambiguous (e.g. Breaking Bad's 2 main protagonists, various GoT roles), they are always inherently likable and you end up rooting for them. I guess it is the British sense of self-depreciation that really allows us to present and identify with truly irredeemable characters (think Basil Fawlty or Frank Gallagher).
So regarding Martin Compston's role - you are not supposed to like DC Steve Arnott. From the opening shots of series one, we begin to realise he is a man of limited ability. While he had a lot of action with the ladies, he is never portrayed as a smooth Lothario, but rather as an insecure and sleazy sex-addict, for which I think a 5 foot 6 average-looking Compston is quite well cast.
This all helps to build the suspense - who do you really align yourself with? Maybe DI Denton is really the soul of the series. Or is it Ted Hastings? No, he is controlled by his financial irregularities. Maybe it is Kate Fleming? Oh - she is also flawed. You always end up second-guessing yourself throughout.
This is quite clearly the best thing on British TV in quite a while.