Stannis is indeed a huge part of the series. He was mentioned quite a bit in the show, but only Renly was seen. Someone best described him as "The Al Gore of Westeros," and that's not far off. He's morally rigid, and a brilliant military tactician, but he is the opposite of Renly: he's loved by no one. While his claim to the Throne is far more valid than Renly's, Renly is by far more popular and able to amass a larger army. These aren't spoilers by the way. The second book is entitled "A Clash of KINGS," for a reason. With Robb King in the North, Joffrey sitting the Iron Throne, and Renly and Barratheon laying claim to it (plus one more mystery guest to be introduced later) the second season is truly going to see war.
I think the casting is a solid one. I always envisioned Stannis as a bit beefier, and it will be interesting to see Dillane with a shaved head and dark beard. What's more important is his chemistry with Melisendre. The chick they go tot play her looks pretty spot on. The character I'm most interested to see cast is Davos Seaworth, The Onion Knight. He's a pirate who was Knighted by Stannis and has a pretty interesting arc.
With all of this said....just like Azkhaban was arguably the best book, the 3rd book in A Song of Ice and Fire, A Storm of Swords is arguably the best in the series. Holy. Shit.
Eddie Doty