Ok first of all, I love GOW and I go the forums every day etc and I would say I know quite a lot about it, not trying to brag but w/e.
Mark Rein said the game will be ATLEAST 12.5 hours on easy for a normal person, it could easily go up to 15 hours. Then theres co-op, which is EXACTLEY the same as the single player one.
Plus when you finish Casual or Hardcore setting (any of them), you will unlock Insane mode.
You're also forgetting this is a SHOOTER not an RPG, shooters aren't very long and I'd rather have a shorter, amazing experience than a long, 25 hours borring and repetative game. And you said you don't like companies which put MP infront of SP. Well this is DEFINATLEY not the case with Gears. CliffyB even said 70% is on SP, and 30% is on MP. This game is more SP oriented than MP. They even have the guy who wrote the Halo books writing the story for Gears.
And you have PDZ, thats pretty damn short.