General thoughts on the Klingons' appearances over the years:
My favorites are the Next Generation/DS9 variety where different families have different ridges. It was a neat little detail, for instance Worf's brother Kurn and his son Alexander had the same type of ridges as Worf, and members of the Duras family all had similar ridges to one another.
I always thought the Enterprise explanation that genetic tampering resulted in smooth headed Klingons was the writers trying too hard to explain something that was best left unexplained. The seed of that is in the episode Trials and Tribbleations where they sort of have to acknowledge the fact that Worf and the TOS era Klingons looked different. Better solution would have been to leave Worf on the station for that episode and pretend the TOS Klingons didn't look any different. Roddenberry himself had stated, when asked if the Motion Picture Klingons were a different race than the TOS Klingons, something along the lines that they were all the same species and race and it was best for the viewer to simply pretend the TOS Klingons had ridges all along, but acknowledge that wasn't possible with their budget. We didn't need an entire Enterprise multi-part arc to explain that visual discrepancy. I personally would prefer an explanation like, smooth headed Klingons were simply a different subspecies, and to have had the guest spots by Kor and Kang and Koloth on DS9 with them showing up with smooth heads. However, the fact they show up with bumpy heads to me is proof that we should suspend our disbelief and go with Roddenberry's answer that the TOS Klingons should've looked that way all along.
I never particularly cared much for the TOS Klingons anyway. Call it blasphemy. That show should've focused on Romulan plotlines more, but I suppose those ears were expensive to produce for a lot of extras.
The change in Romulan appearance bothered me too. They go from looking like Vulcans to having distinct forehead ridges to looking like TOS Romulans again (albeit tattooed and bald) in Abramsverse. I think the ridges were always unnecessary since it had been made clear they were a very closely related offshoot of the Vulcans. I'm just glad there wasn't an Enterprise season 5 because they probably would've come up with some stupid explanation that the ridged romulans were the result of interracial breeding with Klingons.
I thought the Klingons in Into Darkness looked weird, but not too bad--still, they should've stuck with the "classic" TNG look.
The new ones look horrid, like some sort of vampire creatures out of a bad horror fantasy movie or something. Even the costume on that guy looks stupid. He looks cartoon-y. I have a very bad feeling about this new Trek.
I like the Viking-Mongolianesque TNG/DS9 era warriors; both aesthetically and culturally, they were the most interesting version of Klingons in Trek, far more interesting than the TOS era ones which were basically a very loose representation of Asian Communists and were completely boring cardboard cutout baddies
Some retconning is acceptable but there comes a point when you shouldn't fix something that was already perfected, and sometimes with a franchise spanning so many decades, it's easier to just ignore certain discrepancies or pretend they never happened, as the explanations are often more problematic and create more confusion than intended. Most of the visual updates in Abramsverse have been a disappointment. The new Enterprise was garbage, it's like the VW beetle, you don't need to tamper too much with a classic design. It looked top heavy with the giant nacelles and the interior was too bright and shiny. I hope Discovery follows a different design aesthetic that is both an update and homage to the TOS era