I have to say, I'm quite impressed. REH's original story, while powerful, is painfully rushed. "Conan meets Bêlit, they engage in lots of piratey plunder offstage, Bêlit is killed by a giant winged ape, the end." Roy Thomas had the advantage of a year or more of comics to develop the Conan/Bêlit romance and the supporting characters therein, so that when the crew of the ill-fated Tigress are wiped out (as anyone familiar with Conan lore knows they eventually must be), it actually hurts to see known characters massacred. And it hurts Conan, too, in a way that few of his various trials and tribulations ever even touch him.
Conan is forever having his ambitions dashed, escaping by the skin of his teeth with little more than a sword in his hand or possibly less ... the fact that he picks himself up, dusts himself off, and starts all over again shows a tenacity of spirit that is more impressive than his famed mighty sinews. But in this case, we get a glimpse of what toll that must take on him after time. Conan's famous amorality may be as much a symptom of shell-shock as it is simply his makeup. When your plans get dashed again and again (including your one great love being murdered by the degenerate descendant of a pre-human winged race), it quickly becomes hard to live for anything but the moment, I would think. I suspect it isn't until Conan actually claims the kingship of Aquilonia that he again develops any kind of human feeling for his companions, in the form of a sense of responsibility for the lords of various Aquilonian provinces who support him against various uprisings.
The other thing that impresses me about the Roy Thomas works is the respect and attention to detail he pays to Howard's work. In just this one volume there are several examples of Howardian lore being used to drive or at least inform the story, such as the introduction of the sign of Jhebbel Sag (an offhand comment in Beyond the Black River that Conan learned it from a kushite inspired Thomas to devote a whole story to the event during this time with Bêlit raiding the Black Coast).
Of course, Roy Thomas then went on to co-write the train-wreck of a script that is Conan the Destroyer, which to a REH purist reads like someone took several works of Howard, de Camp, Lovecraft, and Chronicles of Gor, tossed 'em in a blender, then threw out anything that required too much thinking or would get an R rating. (With the exception of the horrible "drunken campfire" scene, it's an enjoyable movie -- but other than name-dropping has absolutely nothing to do with Conan.) I'd be very interested to hear what he had to say about that.