I played Space Marines, and now play Imperial Guard, because:
Lots of meatshil... er, Men.
Lots of heavy weapons
Lots of Special Weapons
Lots of bullet spong... I mean men.
Lots of Tanks
Lots of GOOD tanks
And lots of Conscripts.
And of course, commissars.
Wait.... Did I mention lots of men?
You can kill one.
You can kill two.
You can kill three, or four, or HELL, even a dozen.
For all I care, you can kill thirty.
Ahhhh, disposable minions, is there anything they can't solve?
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As far as stories go, I really like the Last Chancers series by Gav Thorpe, and the Horus series by multiplle authors.
Oh yeah, and Eldar Suck.