Overall, I think the show's done an excellent job of reinterpreting the books onto the screen, and most of the changes in Season 2 make sense IMO given the difference between the mediums (and the budget). If there was one thing that disappointed me, it was that they let Robb's and Catelyn's relationship erupt into open conflict over her setting Jaime free. The way he forgave her for doing so only to use that gesture to corner her into accepting his change of marriage plans was one of my favorite scenes in the book, and I was disappointed to see the TV show go another way. Stannis and Melissandre having sex was also completely unnecessary in my opinion, and only served to cheapen Stannis as a character. Other than that, it was good. Even Ygritte came off as an interesting character, whereas in the books she was just annoying.
On the subject of Renly's and Loras' relationship, it's never explicitly described in the books, but it's still fairly obvious. On one occasion Jaime even threatens to take Loras' sword away from him and "shove it into a place even Renly never found."