Shockwave: An insane Mad Scientist trying to help the Decepticons kill all Autobots or a Well-Intentioned Extremist who's doing all he can to actually save Cybertron while trying to appease Megatron for more resources to do so.
Starscream: A traitor who wants to rule the Decepticons because of his Pride? Or an abused underling who has deluded himself into thinking he is the savior for his world and desires to make something of himself before the planet dies. He genuinely seems like he doesn't really want to fight the Autobots at all. Instead, the Decepticons focus on rebuilding their once great capital in his image with no desire to waste energon continuing the war. Arguably, Starscream no longer cares about the conflict once Megatron dies.
In addition, was Starscream really believing his own hype? Or was he hating himself for constantly digging himself in deeper? The look he gives Megatron's body before declaring himself leader may be him contemplating if he should, because he knows there won't be any turning back.
Author's Saving Throw: One of the elements fans expressed the last game as lacking, was that Optimus and Megatron never physically encountered each other once during the events of the story, and thus they never had one of their famed one-on-one confrontations. In this game, the final battle is Optimus vs. Megatron, with the option to play as either one, in a swordfight to the death as the Ark crumbles around them.
Awesome Ego: Throughout his playable sections, Megatron makes several boastful comments and he can back up every single one, especially while fighting in the gladiatorial arena.
Big-Lipped Alligator Moment: The Lever Transformer that appears during Jazz's chapter. You attempt to use a lever, it transforms into a robot that dances around for a minute, a door behind it (containing a lot of Energon shards) blows up, and it transforms back into a lever.
Character Rerailment: Invoked with Grimlock. The developers point out that he was always among the most powerful transformers, but became a joke character as the original show went on. Using his introduction in the game to reestablish him as the monstrous powerhouse he's always been.
Demonic Spiders: Shotgunners. Fast, heavily armored (even with one-shot kill weapons, like the Riot Cannon, they take two), and more than capable of getting up in your face and executing you with a point-blank shotgun blast before you even know what hit you. They also have an annoying knack for sidestepping headshots if you try to snipe them.
And that's to say nothing of the Guardians in Cliffjumper and Starscream's levels. Identifiable by their gray plating and green lights, they rove about looking for an enemy to blast. The best way to kill them is to cloak (there's a reason they only appear when you're playing a character with that ability) and sneak up behind them, then perform a quick execution attack. But if you draw their ire, then welcome to die. They transform and pummel you with a brutal series of blasts more than capable of smashing your shields before you even know what's happening, and even if you re-cloak, it becomes very difficult to get them to go back to their docile form. They're killable, but it's an endeavor. Taking on one is antsy but doable, but taking on a squadron of them is suicide, full stop. And yes, they will chase you into different rooms. Thankfully, they can't follow Cliffjumper into the vents nor can they follow Starscream if he flies off. However, in both levels where they feature, there are also the V.32 Corruptor gun, which turns enemies temporarily into allies...
It's also worth noting that these guys also show up in the co-op escalation mode, where you'll almost always be fighting them in their "attack" mode.
Ensemble Darkhorse: Metroplex. The fact that he kills Megatron only makes him even more popular.
Even Better Sequel: It takes everything that worked in War for Cybertron and refines it into a more narrowly focused thrill ride that puts you into some amazing set pieces. While War had a decent story, the co-op campaign forced them to generic-ize the characters and their abilities. This game actually puts you the the controls of a specific character and you get some very dynamic experiences because of it.
Game-Breaker: The Thermo Rocket Cannon with its firepower and lock-on/homing abilities, so much so that the first patches to the game nerfed it. Also the Riot Cannon, which is powerful even before the upgrades.
Genius Bonus: The Earth in the holograms does not show a contemporary Earth. It's in the process of forming the modern continents as Earth really was in the twilight of the dinosaurs.
He's Just Hiding: Grimlock. Not many fans believe that he really died when the space bridge went down.
Soundwave being the only Decepticon crushed by Megatron's death and providing everything he can to revive him.
Iron Woobie: Optimus Prime as always. Despite seeing so much death and destruction from the war, his Iron will gives him strength and he maintains his optimistic hope for a brighter tomorrow.
It's the Same, Now It Sucks: Not in the sense that it's like War For Cybertron, but like the video game tie-in of Dark of the Moon, which had single-player levels that "focused" on making the playable Transformers unique. Hopefully, High Moon will manage to cure this problem.
To note, it was the removal of co-op that got to most people.
Although some folks changed their tune when they learned that said tie-ins would result in the Dinobots appearing in Prime.
Jerkass Woobie: Starscream. He's thrown out of the Decepticons after Megatron and the other troops finally have enough of him. The sad part is that he has nowhere else to go and even stays hidden in the base so he can get even with the troops who abandoned him and sabotage Shockwave's projects. This is heavily supported by listening in to all the troops mocking him while they think he's gone for good.
Memetic Badass: Grimlock. The trailer made him even more awesome before his gameplay was shown.
Memetic Mutation: "METROPLEX HEEDS THE CALL OF THE LAST PRIME" and variations of it have become popular with Transfans.
When Optimus sees that Megatron is as desperate for energon as the Autobots, he proposes a truce to resolve their mutual problem. Megatron refuses, blames Optimus for everything, and insists that he alone is Cybertron's future, swearing to kill Optimus and the Autobots.
Shockwave's experiments cross this, as he surgically rips apart and rebuilds whoever he experiments on. To make it worse, he keeps them functional and aware while he tears them apart and rebuilds them. While most of the Dinobots seemingly don't remember, Grimlock does.
Most Annoying Sound: "Hurry Grimlock, Snarl can't take much more of this!" It wouldn't be so bad if not for the fact that he says that almost every, single, time, Snarl loses health. Also, Sharpshot's taunts whenever Grimlock takes damage, which is likely intentional.
Paranoia Fuel: The ruthless, murderous, treacherous schemer has a cloaking device and is very good at sneaking around. Meaning he could be anywhere, at any time. He could be right behind you, listening to you, ready to run you through with his sword, and you wouldn't even know it. How do you know he's not behind you right now...?
Polished Port: While the PS4 and Xbox One releases don't do much more than put the DLC on the disc, it does have fewer technical issues than either the PS3 or Xbox 360 releases, with a more consistent framerate, for one.
Porting Disaster: The PS3 version has some issues, although it's still playable. The framerate gets rather low during the most intense scenes, textures load rather slowly resulting in low-detail characters, the game occasionally stops to load during gameplay and cutscenes and the game has a tendency to freeze/crash.