Ok here's my thoughts on the game:
1. It is very short, I finished it in 8 hours. A person new to the game may find its life expectancy to be around 15 hours at most.
2. Very polished single player, but it gets monotonous at times - kill monsters, meets locked door, go find switch to open door. Vehiclies are also pretty fun.
I think it had a lot more potential in the 2nd half of the game, when you turn into a strogg. The developers could of added so much more extra abilities, but all you get is the ability to read strogg, run a bit faster and use strogg medical stations.
3. Fairly straight forward, think all the good qualities of Doom 3, add in open terrain, fellow space marines and change the monsters to cyborgs to boot. Highly scripted though, very nice voice acting.
4. Its fairly easy, I thought many monsters will be uber bad ass, only a few are unfortunately. Bosses are very easy. I didn't die once against the last boss.
5. Your allies kick ass, think along the lines of marines from Halo and soldiers from Half-Life opposing force, but better, some can pack just as much punch as you do. Medics and engineers are invaluable, unlimited health and armour!
6. Nice weapon mods, like clip holder for the shotty or tracking nails for the nail gun (which is unstoppable). The rail gun however is a pee shooter, its not lie quake 2 where its a massive slug which kills enemies in one shot, it's like a streak of laster which takes 3-4 shots, even with the weapon upgrade. Hyperblaster is a bloody killing machine, almost instanteous hit, very fast firing rate and does heaps of damage,
That's all I can think off atm.