From what I understand the Blood Ravens were invented by Relic because GW wouldn't let them use the Ultramarines for their RTS game. So it seems like Relic was set on using the Ultramarines in a game from the start.
Ultramarines have a pretty neutral personality, so they were the ideal chapter for introducing the Warhammer universe to the mainstream video game player(who don't even know what Warhammer is). I think the main problem is that there just wasn't enough dialogue in general, every time you pass through an empty area was a chance to have a conversation. There could have been more said during battles too, in fact the Dawn of War games all had a lot more sounds to their battle besides gunfire and shouting. Space Marine could have used a little bit of that Dawn of War polish.
A generic story flow isn't a bad thing if you have decent dialogue. If the game can entertain you with battle scenes, and then entertain you some interesting/funny/dramatic dialogue scenes inebtween, then the game has succeeded in entertaining you throughout the whole experience.
Relic may have taken the script for granted, assuming the player just wanted to get back into the fight fast.
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