Okay, instead of citing episodes and movies, I'm going to say this: Klingons in TOS looked human because they didn't have the budget or technology to make them look different. Klingons in everything else look different because they wanted them to look scary. Done. And yes, the Klingon in ID does have the ridges in his forehead- he even has piercings in each one (which is cool). But the retcon they came up with to explain away this obvious difference doesn't always work.