The commentary you give on the cards (called flavor-text btw) is usually set italic; Put it in [ ] to do so.
Just some general rule and/or syntax-related corrections:
1) There is no need to give the Doom Marines ability a keyword-name. Also the ability itself is just odd. Other than that it is Toughness, rather than Defense, MtG-cards usually don't write them out. Instead they are displayed by a simple "+x/+x"-thing, where x is the change to the creatures stats. Additionally, you can not just switch between applying bonuses. But since you want to simulate a close-combat-frenzy that doesn't affect its ranged-attack, why don't you add a drawback against the only creatures it needs to fight witch reach? Or have it lose its reach ability? Its kind of tricky to pull off this mechanic; All my solutions end of up with weird syntax with like three instances of "until end of turn" in one sentence.
Just make it "{3}: Doom Marine gets +2/-1 and loses Reach until end of turn."
2) Sam's ability would be written out as "Serious Sam enters the battlefield with X +1/+1 counters on it, where X is equal to a third of the total number of creatures on the battlefield"
3) Your card frames are kind of chaotic. Just remember: A card always has the same framecolor as the color of its colored manasymbols.
With more than one sort of mana in its cost, the frame is golden.
4) The "basic" prefix is unnecessary/not used on creatures. The only thing basic are Basic Lands.
5) To guarantee versatile usability of your cards, especially in combination with existing cards/custom cards that follow close to original cards, try to emulate the same creature-type rules.
In this case, write Zombie, instead of Undead, and Human Soldier, instead of just Human.
6) What's with the Reach? Other than the fact that Reach is almost exclusive to the color green, it is a blocking mechanic, that is also considered on the weaker ranks of MtG-mechanics.
That's all. Well, close to all.
One question: Why did you post this in unglued?