Loras in the novels is pretty much a Tyrell version of Jaimie, emphasis on Jaimie's martial skills and more knightly virtues (bravery, boldness, dedication, and dangerous fighting abilities). And to use a TV Tropes phrase, he, like Book!Renly, is also more of a "Straight Gay;" a person whose manner of carrying themselves and interests fit the heteronormative stereotype, but still has a romantic focus on the same sex. He's almost literally Jaimie 2.0, but further back in his story and with a different social taboo; he joins the Kingsguard to protect his sister, has a powerful and overriding loyalty to his love, is somewhat defined by how dangerous of a horseman and jousted he is, and even his hubristic mistake in the last book is a macho badass move. Jaime actually warns him about the potential mistakes he could find in his his way, but still acknowledges that without his own sword hand, Loras might be the most dangerous and skilled warrior in the Kingsguard.
Contrast that with TV!Loras, who, after the Tourney of the Hand (which in the TV show left out his previous impressive victories without the "mare in heat" trick) and his actions in Season 2, never really has his "manly" traits emphasized again, and seems to play more to a stereotype of what a homosexual would be at the time.
Please note, I'm an evangelical straight guy, but that's how I see the difference. I will admit that it's conceivable they wanted to showcase a more openly gay individual to out modern eyes, but I can't comment on whether or not that was the right choice.