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Klingons were nearly featured in Abrams' Star Trek and now live only in a deleted scene where Nero and his men serve time on a Klingon prison planet. Much like the cover of the upcoming IDW comic though, they are seen only with helmets.
Fans of the original Star Trek series already know that, due to budget restraints, the Klingons were simply human looking with dark makeup and fancy facial hair. As the movies and future shows moved on, Klingons soon looked like the Warf we all know and love. Though several writers attempted to explain the difference in their own way, the show Enterprise attempted to officially address this change within the Trek universe with the shows "Affliction" and Divergence".
A simplified version of the explanation is that the Klingons were doing genetic testing to create more powerful warriors. First, the tests were not completely successful, though they did cause changes to their personalities and appearance, not only giving a more human look, but also saddling them with many human-like emotions; it also resulted in agonizing death eventually. Second, one of the subjects was also carrying a type of flu that combined with the genetic changes and began to spread among the Klingon race unchecked.
Gene Roddenberry, Stark Trek's creator, never felt an explanation was necessary. To him it was a simple budgetary issue and they did they best they could with the alien races in the 60's show. Combine that with most fan's attempts to block Enterprise from their memory and that 'official' explanation might not matter so much.
So the question now is, how will Abrams handle this in his Star Trek universe? The helmets make for a good tease, but certainly if Klingons are more prominantly featured they will need to address it in some fashion. (A still from the Into Darkness trailer featuring a Klingon in battle below) IDW's comic, we've been told in the past, is considered canon for the Abramsverse, but I'd be surprised to see the helmet's come off in the comic before they do on the big screen. You never know, the prequel comic for Star Trek definitely added a lot to the story and will hit stores in April, only a month before the movie comes to theaters.