@Mazty: Again, read the fluff. Marines do not conquer worlds in Warhammer 40,000. They take out commanders and then let the ground troops - regular humans - do the rest of the work. Marines are only good as:
1. Highly elite shock troops, i.e. in a supporting offensive role to the Guard.
2. Destroying the enemy command with a single decapitative alpha-strike, which rather relies on your opponent not being unassailable (i.e. Vraks), or having a decent command structure ("Oh, dear, General Logos is dead. Promote Major-General Tacitus), i.e. will only work against incompetent enemies. Even then, you still need human soldiers to do the actual work of attacking, occupying and mopping up the enemy forces.
3. One marine vs. a world? That's some great propaganda there. Or was this world inhabited solely by peasants and small children?
4. Okay, let's put it this way: Space Marines are 7' 8" tall supermen in massive armour? Badass?
Yeah. Except Tyranids have creatures upwards of 20 feet tall that can eat dozens of marines. For breakfast.
Eldar have ancient warrior mystics with hundreds of years of fighting experience, indeed, thousands of years if it to believed, warrior mystics who can change the very course of time and space, and firepower so powerful it rips power armour open like it's tinfoil.
Necrons doubly so.
Guardsmen face down the worst horrors of the galaxy with a lasgun and flak armour. Oh, and heavy weapons capable of levelling continents, tanks the size of buildings, elite commando troopers, hardass jungle fighters....yeah. Oh, and there's Commissar Yarrick, Colonel Straken, Guardsman Marbo and, of course, Creed, a man so tactically inspired he cannot be outsmarted.
Tau...well, meh. Good tech, bad philosophy and design aesthetic.
If one is entirely honest, the Space Marine is one of the more boring parts of 40K. He wins because he's Action Man, a one-dimensional hero with no character, a darkling superman with who's main appeal is to out desire for 'Der Ubermensch'. You've got a universe full of hideous monsters, dark, sinister conspirators, chainsmoking, vicous commandos, savage aliens, daemonic legions....and to send against it? Superman in blue armour.
Bah. Send me a Guardsman any day. At least he'll know how to outsmart his foe.
