The way I see the series...
Season 1 was an introduction. Introduce us to the characters, the overall story, let us get comfortable with what is giong on and what everyones motives are.
Season 2 was all about character development. I agree it was rather bland, but I feel it was just all about really showing who everyone was and watching them grow as a character.
Season 3 was where everything came together. You get to see how everyone has coped with all that has happened, and get to see where certain people stand.
Sure, it has had a lot of filler and plot holes, but I still think it is one of the best and most interesting shows that has been on in over many years. I really loved Season 3, I have been hooked on it, and I thought the season finale was great as well. It didn't leave as much open as the Season 2 finale did, but I am still hooked.