Starkiller's signature lightsaber grip is shared by just one other major character in the official canon: Ahsoka Tano. Both trained under Anakin Skywalker. Coincidence? Perhaps not. It is quite possible that after seeing how destructively Ahsoka used the style Vader trained Starkiller in it.
Technically what Ahsoka and Starkiller are using is called Reverse Shien due to the reverse grip on the lightsaber.
Aditionally, why is Vader so sure he could beat Obi-Wan on the Death Star? Aside from the Dark Side, he already has, when PROXY takes that form
The romance sub-plot. Early on, Galen goes from dark and brooding while kneeling in front of Vader to a gibbering teenager who hushes his best friend around a corner to talk about the hot chick like there's been an Unexpected Genre Change to teen highschool drama. Later on, he goes from supreme badass to wide-eyed and awkward when she kisses him out of the blue, and General Kota makes a big deal of how his feelings for her helped turn him good, despite the kind of person she must've been to have risen in the ranks of the empire enough to get Vader's attention. Except...consider how he grew up and it's not so unbelievable, since normal, healthy interaction with other people isn't exactly part of Sith training. Does Vader, of all people, seem like the kind of guardian who would bother explaining to a young man growing up in his care that the funny feeling in his stomach (among other places) when a pretty girl is around is normal? She's probably his first kiss, if not the only woman in his life he hasn't either killed as an early target or on the way to one.
It also parallels Anakin's life. No wonder Vader would stay away from discussing such things.
Another thing about the differences between Anakin and Galen's upbringing. Anakin was pampered by the Jedi Order, told that he was The Chosen One from an early age, and given special treatment compared to other prospective Jedi his age because of it, leading him to become spoiled and arrogant later on. Galen, on the other hand, was constantly belittled, manipulated, and betrayed by Vader, who constantly told him he would never amount to anything more than a tool despite his power. Abusive as this was, it also kept Galen humble and pushed him toward those who could give him an escape from Vader's cruelty (Juno,Kota, and the Rebellion), facilitating his eventual turn from the Dark Side to the Light. Ironically, Vader's Training from Hell made Galen a better jedi than Anakin Skywalker ever was.
In the second game, Vader uses Starkiller's love for Juno against him by using her as leverage to join the Darkside. That's right, he's using the very idea that turned himself over to Palpatine against Starkiller.
Vader's whole Juno plot, specifically after he "kills" her is pretty complicated. He threatens to kill her and makes Starkiller pledge his loyalty to him, similar to Sidious and Padme. But then, by hurling her off the platform, he sends Starkiller into a massive, unstoppable rage. He then gives him advice to calm down that sounds like Yoda's lines from III about Padme. So it appears that he's using two different methods to turn him to the Dark Side, then uses reverse psychology (and then throws the fight to destroy the Rebel Alliance from within, but that's beside the point. Good work Vader.
Why does Starkiller's lightsaber have an exposed crystal chamber (something seemingly impractical, as the crystal could get dislodged in the middle of a fight and screw Starkiller over)? Being a video game character, he wants to be able to change his crystal on the fly, since he finds many other crystals in the overworld.
The best way to defeat Vader in the last level is by cutting loose and being as aggressive as possible. It reminds one very much of Luke's final battle with Vader in Return of the Jedi.
And the best way to get through the fight with the Emperor is to reflect back his attacks and fight defensively. A more light side way of fighting.
The fact that the player can use Starkiller's aggressive version of the Jedi Mind trick over and over on entire platoons and troops will keep falling for it, despite the fact that they're witnessing their fellows kill each other and themselves time and time again, yet somehow the trick keeps fooling them. It's because they're weak minded right? The REASON they are weak minded is that they are rank and file soldiers used to being told what to do and then doing it without question, no matter how absurd, suicidal, and morally wrong it is. Notice that the more elite troops don't fall for it, and that's because they're used to giving orders more than taking them.
Copying swordfighting skills is one thing, but how come Proxy can use Force powers while impersonating a Jedi? On that note, if there is a robot that can copy a Jedi's abilities and powers exactly, then why hasn't anyone build an army of them yet?
The novels say that Proxy can use tractor and repulsor beams to simulate telekinesis. It's not as good as the Force.
Also, remember that Proxy is a personal droid of Starkiller's designed to be a protocol droid, message droid with extremely high resolution, and combat training droid designed to be able to match Force Users blow for blow while being made of lightsaber resistant materials. Droids like Proxy are probably ludicrously expensive to make and thus too impractical to make large numbers of, even if one cuts corners.
Proxy's heroic sacrifice in the first game is treated dramatically, but why can't Starkiller just rebuild him?
Because they were in a hurry to rescue the senators and Kota. There was no time. In the sequel, we see that Juno did eventually go back to recover Proxy's remains and rebuild him.
During the final battle in the second game, some of Vader's taunts are truly terrifying once you realize they could be applied to him as well: "The woman is meaningless! She is holding you back." and "Rise above this. She means nothing!" really show just how little is left in Vader from once loving husband.
Using the Salvation to attack the clone facility is admittedly awesome, but just how many Rebels who'd managed to survive the initial attack died during the impact?
You have to read the background information a bit but the unit assigned to to guard the cloning facility? It's the 212th Attack Battalion, lead by Commander Cody and Obi-Wan's old unit. They are utterly wiped out when the Salvation hits and if you read some of the databank info, the 212th was miserable on Kamino. Really the attack just seems like the end cap on an very crappy tour. It's also a bit heartbreaking that an character, while not quite beloved, was still very much someone people who'd watched the Clone Wars had grown to like and he's just killed by having a ship dropped on him? This is a man who'd survived the Clone Wars, the turmoil of the Rise of the Empire and he dies offscreen. He was a minor character in the grand scheme of things but still....