I always thought Spock looked a little too human also (it was the 60's after all. And I believe his father Sarek made this comment in a flashback in Star Trek VII the undiscovered country?). In TNG and DS9, Vulcans looked a lot more distinct. Romulans ALWAYS looked great.... Even Mr Nimoys own makeup (for the TNG cameo's i remember) made him look distinctly LESS human.
Not sure I buy the genetics thing. I dont expect her to be decked out with a house of martok bandolier but even B'Lanna's demeanor was pretty "neuterred" (i really cant think of a better word.. sorry!!)
Also, Re: your genetics comment, (and I know this is a bit off topic), but i start to think about Alexander Rozhenko who was a quarter human I guess? As a child, he looked very distinctly human but seemed to get more klingon as he aged into a young adult.. (he developed the klingon incisors for example). So maybe you are right. Your mileage may vary
* The Voyager non-humans looked really.. well.. HUMAN. Kes might as well have been a chick I dated in highschool, Tuvok looks more human than vulcan, B'Lanna is like Meg Griffin with a head injury (look at her compared to Kaylar)
B'Elanna's father was Human while her mother was Klingon.
K'Ehleyr's father was Klingon. As you can guess, her mother was Human.
In genetics, that can make a difference in how prominent a trait or two turn out. The development of the cranial ridges in Klingons be more expressed through the male sex chromosomes or other traits passed down from the father. It would explain why B'Elanna had ridges like that.
P.S. At least several Vulcans before Voyager came out looked a lot more Human than Tuvok did...