I think they'd have been hard pressed to find a better actress to play Avasarala. My only gripe is that she doesn't swear enough.
So far, I'm still liking the earlier inclusion of characters (for those who haven't read the books, Avasarala and Bobbie didn't appear until book 2). Their appearances make sense, and provide a little more backstory to those characters as well as depth to the series.
This is how you adapt a novel series to TV. Season 1 and 2 are going to be Book One, Leviathan Wakes. And with six (to date) books in the series, that leaves a LOT for the TV series.
Remember, doors and corners. That phrase carries great importance going forward.
Oh, and I miss Miller's hat. :(