This question originally appeared on Quora. 
Answer by James August, Student
Ages
Television regulations influenced casting for characters who are much younger in the novels than they are represented in the television series. The Stark children save for Robb and Jon are all up-aged, and Robb and Jon are supposed to be 14 and 15. Margaery is 16, and Daenerys is 14.
Loras/Renly
Most people object to the homosexual overtones because there seems to be no basis from the novels. But closer reading can lead to a different interpretation of Loras Tyrell and Renly Baratheon's relationship. *SPOILER* Loras is deeply affected by Renly's death, and when Sansa brings up the subject, his response is similar to someone whose cause of grief is misinterpreted. Also, GRRM himself is a consultant, writer, and producer of the series, so if Loras and Renly were not supposed to be gay, then he would have said something.
Histories and Flashbacks
In script-writing, flashbacks and backstories are difficult to incorporate because they can weigh down the action and the bulk of information is not vital to plot progression. Luckily, histories of the houses and the conquering of Westeros can be seen in the special features of the DVDs. In the novel, this information is given when people talk about the houses or when characters emphasize that their motivations are part of the traits of their house.
Frequent flashbacks are tedious and also break up the flow of scripts. Unfortunately, they give you crucial information into childhoods and in turn the reasons why certain characters will do something or what their connection is to another character.
Sex
Anyone whose watched any other HBO show knows just how important sex is to the company. Granted, GRRM lands a raunchy paragraph now and then, but they're few enough. The prevalence of prostitution is also more evident in the show, but in truth its just as commonplace as GRRM writes it -- it's just that the majority of our POV characters are noble and seem to spend more time trying to survive wars and politics than having sex with prostitutes.
Concurrent Storylines
The novels are written through POVs and linearly. Therefore, whatever happens after a character acts may be put off until a later chapter. I see it as a positive to the show to be able to switch between various characters and various sides of an event so that the audience can understand what's going on a bit better. GRRM used it for suspense and anticipation (and to keep us reading through the boring POVs). In the novels, we only witness just one battle in full -- Blackwater.
Names
As far as I know, only one name has been changed -- Asha, Theon's sister. Apparently they thought the audience would be confused because it sounds like "Osha,",= the wilding woman now in Winterfell.