This movie deserves a 9.5 out of 10 in the "Make fun of the movie with your buddies" category. In the real world of motion pictures, a 1 out of 10 is generous.
If you've gotten to this comment, then you've read multiple times by now how the main character is "bullet proof" maybe "flame retardant" or even invincible. Well, that's all true, but I expected that going in. There are far larger problems than that.
So, here are the top 5 problems with "The Marine."
5) Cena is NOT a badass in this film. He isn't a good fighter, he doesn't have any tricks or intellect, he's not even good with guns. He's a big, goofy jacked dude who ends up winning every fight despite getting his ass kicked for most of the movie. You don't "connect" with him at all. You don't see a bad guy do a bad thing and think "Oh man, I can't wait until Cena gets a hold of him." That never happens because Cena is never established as a badass in any way besides just being jacked.
4) You don't love the hero by the end, and you don't hate the villain. Robert Patrick is actually level-headed and likable as the villain. John Cena has no personality, no character, and no endearing qualities (besides just being un-godly jacked).
3) There are several subplots that are terrible, yet un-finished. You'll see multiple head-scratchingly bad conversations between characters, and when they're over, you'll ask, "what was the point of that?" In the end, you see that there was no point. No foreshadowing, nothing. Just stupid meaningless conversations that trick you into thinking they were important to the plot.
2) The main character does not change, or learn anything, or accomplish anything. He's completely static.
1) There's no reason for it to be called "The Marine." He pretends to be a marine for less than 5 minutes of the film. The rest of the time, they just repeat that he was once a marine in order to jam it down your throat that he's tough.