That Aliens: Colonial Marines E3 demo sure was neat, but you know what wasn't neat? The actual game. According to an anonymous source that spoke with Kotaku, this was always going to be the case since the demo ran on hardware that far outmatched the power of current gen consoles.
"We were told many times through demo production, 'Don't worry about performance, just make it awesome,'" said Kotaku's mysterious source. "There was a reason [the demos] were never playable."
According to Kotaku, the E3 demo was built by TimeGate with animation by Gearbox and it ran in real time. Due to budgeting and the realities of current gen hardware, the development team (jury is still out on who exactly that is; TimeGate or Gearbox) had to cut back on visuals for the final product and don't the results show:
"We were constantly cutting back more and more in terms of texture, shader and particle fidelity, in order to fit into the jacked memory restraints," said a separate unnamed source.
And that is how you go from "wow" to "2.5."
From Dream To Disaster: The Story Of Aliens: Colonial Marine [Kotaku, image by VideoGamer]