For as well-developed a queer character as Oberyn was during his brief time on the show Loras sure wasn't. The "Knight of Flowers" is actually the focal point of another major deviation from the books. Namely, the rise of the "Sparrows" in King's Landing.
In the books these religious zealots were the result of years of oppression by the rich and powerful. Basically, they're the workers' revolution -- rising up against the medieval bourgeoisies. It's a decidedly left-leaning political movement at the start. One that ultimately winds up directing its ire at women rather than gays, and even then not to such a degree.
Perhaps that was just too multi-layered for the Game of Thrones showrunners, or perhaps they simply needed something for Loras to do. He certainly didn't have much between then and when his lover Renly got the axe. Besides defending King's Landing against Stannis in Renly's armor, of course. We'll give them that one.
From that point forward, and up to the Sparrows arriving on seen, he existed pretty much only to look limp-wristed next to Cersei. Every once in a while she'd tell him to shut up, and he'd do just that. When came the rise of the homophobes the Sparrows abused him further, throwing him in a dungeon and beating him into submission. Not so in the books, where Loras joined the freaking Kingsguard.
The place of high honor came with a glowing recommendation, too. Jaime Lannister looks on Loras and sees a younger version of himself. Not only that, he realizes that when Loras beat him at jousting years earlier it was no fluke -- the young man was really just that good.
It's worth noting here that in the books Jaime has realized what an ass he'd been up to that point. Freed from his family, and fresh off his time with Brienne of Tarth our boy Jaime is truly developing into his own man. Which is to say his vote of confidence goes a long way. Though it might not have been enough to save Loras from Cersei, who -- the last we heard -- practically sent him off to his death. Though we won't know for sure until Hell freezes over, and the next book comes out.
Oh, right. Jaime. Remember that bit about how Game of Thrones loves to double down on the rape and abuse of women? Well, Jaime's rape of Cersei caused quite a stir when it occurred on the show for no good reason. Though everyone involved shooting the scene didn't see it that way, when it plays out on the screen (Cersei literally says "Stop."), it's hard to see it as anything other than sexual assault.
While the two did go at it next to their dead son in the books, there was no force involved. At least not any the two of them didn't agree on. The gratuitous character moment in HBO's version was, in the books, basically just two lovers happy to see each other again. Albeit with zero sense of appropriate time and place.
Game of Thrones viewers know that after this Jaime heads to Dorne -- where plot movement and characterization go to die.
Here he spends his time basically acting as a proxy for the audience. Because we know Jaime we're fine learning about Dorne! Except since Jaime is there, Dorne has no time to develop. It's boring as hell, and Jaime seems boring as hell by association. Not to mention this is his first and only role after raping his sister, so Jaime isn't the most thrilling person in the first place.
Not one bit of this happened in the books. There Jaime headed north, not south. Specifically to pull the last bannermen loyal to Robb Stark up by the roots. During which time he begins training his still-on off-hand to better wield a sword with the help of Ilyn Payne. That's the royal executioner, in case you'd forgotten.
Since Payne can't read, write, or speak (what with not having a tongue anymore) Jaime intentionally selects him as a sparring partner. That way he can use the honestly indifferent knight as a sounding board for all of his deepest, darkest secrets and philosophical epiphanies.
As previously mentioned, Jaime realized that he's been an arrogant dick for most of his life. He attributes much of it to growing up as a Lannister. This newfound contempt for his family's dickish legacy even extended to his sister. In fact, Jamie is so out-of-love with Cersei in this period that he ignores her when she pleads for him to rescue her from the traitorous Sparrows. This is in stark contrast to Show-Jaime, who wants to do anything he can to help Cersei, and has yet to experience his book counterpart's crisis of conscience.
By the end of book five, however, Jaime might be in trouble. Brienne finds him again at the end, and might just be plotting to kill him for a merciless, zombified version of Catelyn Stark. Yeah. The books get weird.