The orcs description is reasonably good, but it is a mistake to omit the orc horde. I'm not sure why orc hordes are so much more common than hobgoblin hordes (although I'm tempted to blame Tolkein). But when you have tens of thousands of humanoid monsters, they are usually orcs. I think any one page description of the orc "story" needs to address the fact that - on rare occasions - orcs gather in extremely large numbers and pose a threat to society as a whole.
I agree that the gnoll description needs to stand a more on its own (and have less of a weird digression into hyenas), but I think it's alright to reference a more common humanoid (like orcs) to show how they are different. The major piece missing in the gnoll description is that they prefer pack tactics. The cowardly part struck me as a little off, but I think that fits into my conception of gnolls when you think that they prefer to attack with numerical advantage, but will flee if they don't think they can win. At least to me, gnolls are the type of humanoid that would pursue a retreating foe for some time, but -- if someone drops -- they would rather torture (and then eat) a single fallen enemy than bother pursuing the healthy ones.
Speaking more generally, I think it's a good idea that they address how both sets of humanoids treat prisoners. (PCs are likely to meet prisoners, even if they don't become them.) But I thought it was strange that neither blurb said very much about the non-combatants in orc or gnoll society. For example, how do they raise children (pups?)? Are orc or gnoll females (I hesitate to call them "women") combat warriors or non-combatants? I don't need a treatise on orc sociology, but a couple sentences would be nice.
-KS