When you look at the space marines in the movie genre, say between Alien and Doom, there is a very distinct and clear difference. In Alien, they were average guys you could relate to because they appeared to still ACT like people. In Doom, they were a bunch of elitist dildos that thought they were invincible, despite half the team being idiots.
In video games, when the main character is a space marine, usually he is a dedicated man who has years of service to his credit (if not genetic enhancements to make his superior to us "average people") and his persona is likened to a superhero-ish stature. Gordon Freeman could be considered to be an average guy who is thrust into the role of a hero, thereby making him easier to identify with. But who among us can say we know what it is like to be a genetically-modified superhero (aside from the test-tube babies of the 80s)?
Sadly, I think that perhaps space marines are becoming too much of a cliche anymore because of the over-abundance of movies like Alien and Starship Troopers and Jason X. We have a lot of characters from movies and pop culture to draw from, and the stereotype has likened the character to one from an action movie from the late 80s and early to mid 90s. We've just seen it so much that the character doesn't really having any individual meaning any more. We see a space marine, and we already have preconceived notions of what to expect. Would Riddick from Pitch Black or The Chronicles of Riddick be considered a space marine?
In video games, the space marine is usually a guy who is gruff and macho and afraid of nothing and shoots first and asks questions later... sort of like a John Wayne-type of space cowboy. He always wins, he always gets the girl, and everyone thinks he is the penultimate hero. So where is the growth in this character anymore? They all seem to do the same thing, and having a moral choice in a video game adds a little depth to the story, but never much to the character. They will always win and make it to the end of the game, the universe will always be saved in one way or another, and all their previous mistakes are forgiven because they end up saving the day. Look at Luke Skywalker from Star Wars... he was an everyday person who happened to be the son of the most powerful person in the known galaxy. So before he really gets to establish himself as an individual, we are already told he is anything but an ordinary person and that casts a different and heroic light on his character because we believe that he is better than us. While trying to maintain his sense of humanity and not get caught up in his powers, we can't help but think that he is better than us because he can do things we only dream about. Does this make him unlikeable? Not really... he still shows emotion and compassion towards his friends without acting like a wuss or some sort of emo anti-hero. He was a moisture farmer with special abilities, but had he not had the chain of events that showed him his actual potential, he would have lived and died a lowly and boring moisture farmer.
I'd like to see a more down to earth sort of space marine like the ones we knew in older movies. Why do they need some power armor or magic abilities to be a hero? The people in those movies we can relate to end up becoming killed by some rookie mistake or an alien's lunch... we rarely see the little guy get his day. I like space marines, I'd just like to see a different character take on the role as the underdog hero.