-1- Benedict Cumberbatch is playing Khan Noonien Singh
Outside of the Klingons, Classic Trek is quite low on truly iconic villains, but Khan would be the top of the pile. He is as close as Captain Kirk gets to a Joker (though Lex Luthor is probably a more accurate comparison) but what separates Khan from those comic book villains is the singular nature of his existence.
Comic books, TV shows, cartoons, movies, video games... The Joker and Lex have existed in multiple iterations with countless stories being told with them. Khan has only ever been Ricardo Montalban and he has only ever had two stories to his name; the season one episode Space Seed in 1967 and fifteen years later in Star Trek II: The Wrath of Khan. The 1982 sequel is not only the best Star Trek movie ever made but one of the best movies ever made, full stop.
Khan Noonien Singh was a genetically engineered superhuman who rose to power during the Eugenics Wars, claiming over a quarter of Earth during the 1990s (you'd think people would remember that), and heralding an era of the superman. When he was finally deposed, Khan and a group of superhuman loyalists fled the planet aboard the SS Botany Bay, only to be suspended in cryogenic chambers until the crew of the USS Enterprise discovered them over 270 years later. Khan attempts to seize the Enterprise, only to be bested by Kirk and left to rot on the planet Ceti Alpha V. He would return in Wrath of Khan to seek vengeance on Kirk and, in the process, inadvertently kills Spock; perhaps the ultimate act of revenge.
Now, you can tell from his name that Khan is of Indian descent (he ruled over Asia and the Middle-East during his reign) and it was a slight stretch to have Montalban, a Mexican, play the part but he made it work through sheer determination and some substantial make-up and costume work. If a Latin actor was a stretch for the role of Khan, how are we supposed to accept Benedict Cumberbatch, perhaps the most English man in the universe, as Kirk's nemesis?
It also warrants mention that Khan's escape and cryogenic suspension occurred centuries before the massive temporal event that caused the Trek universe to become rebooted, so everything that happens before Nero travels through the wormhole and kills Kirk's dad is the same as the regular continuity. This means that as the sequel begins, Khan should be the same as he was in Space Seed (will we see Cumberbatch in Khan's original get up?) and have absolutely no real quarrel with Kirk or the Enterprise, beyond a need for their vessel.
Look at it this way, it wasn't Space Seed that turned Khan into an iconic villain. Without that fiery drive, what sort of compelling conflict do we hope to see? Of course, we already saw one villain seeking revenge on a Trek staple in the first reboot, wen Nero hunted down Spock, repeating that with Khan and Kirk would probably be boring. Do they have something fresh and interesting up their sleeves?
In Star Trek 2's defence, The Wrath of Khan was a Trek movie that wisely knew that good drama and strong characters should come before continuity, so, while the movie isn't slavishly loyal to canon at all times, it is a fantastic piece of cinema. This is something the Trek franchise lost sight of, over time, which is why it began to alienate audiences. The Trek reboot won audiences back by jettisoning decades of continuity and starting fresh, but it was far from the perfection that Wrath of Khan achieved.
It's good to be open-minded about Abrams and companies potential to create a worthy successor, as being a stickler for those continuity details I mentioned above does not necessarily lead to a better movie for everyone, it's also understandable that people would worry that they have opted for the easiest, laziest choice for an antagonist and that such a decision could reflect poorly on the rest of the film.
-2- The Klingons Are Back
The most famous species in the Trek Universe, outside of Vulcans, and it seems more a question of when, not if, they would appear in Abrams Trek movies. The Klingons were originally slated to appear in the first movie, but were relegated to a deleted scene on the Trek DVD/Blu Ray (viewable here). We see the Klingons (disguised in heavy armour, including a cowl to cover their faces) capturing and interrogating Nero after he falls through the wormhole.
TrekMovie's sources say the Klingons will be given a more substantial role in the sequel, one that cannot be so easily cut around in the editing stage.
-3- Spock Prime Returns
Leonard Nimoy acted as the bridge between the Classic Trek Universe and the Rebootiverse, he was an ambassador of sorts, he let older fans feel comfortable that this universe can co-exist with the one they know and love and that this unique reboot did not invalidate everything that came before. It was a show of good faith and it worked, even if the script logistics of his arrival did not match the gravitas that Nimoy brought to the screen.
Do we need Spock Prime to come back? Of course, at this point he now resides in the Rebootiverse and is helping establish a new Vulcan home world, so he's still out there and could easily reunite with the new crew. It's always nice to see Nimoy break out the Vulcan salute but it feels like he will once again serve as a bridge between the two universes, explaining the long and dangerous alt-history between Kirk, Spock and Khan.
Anything that brings to mind the brilliance of Wrath of Khan is going to work against Trek 2, so I'm once again hesitant. Unless we get a scene between Khan and Spock Prime, that would certainly be loaded with potential drama, as the aged Spock faces the man responsible for his own death. That would be pretty thrilling for any Trek fan to behold.
I liked Star Trek, it was entertaining in a way that few more recent Trek movies managed, even if the script was a mess. My concerns surrounding Trek 2 are that it feels like every creative decision being made are the safest options possible. We are getting the most famous and obvious choice of villain, instead of inventing a new villain or taking a less well known character from Trek canon and giving them the Wrath of Khan treatment. We are getting the Klingons, a necessity for the Trek universe but another obvious choice, and we are seeing the return of Spock again.
The general sentiment appears to be that; if it worked before, it will work again. I hope that is the case and that these safe choices lead to something special, anything less than that is going to be a major disappointment.