Marines
Weak, but you gotta love 'em
The Marines may be weak, and it's quite annoying the way they spend most of their time rolling around rather than actually shooting when under attack, but they're still a highlight of the game, creating great atmosphere and seeming so alive with their superb speech programming and voicing. This is nowhere better experienced than when fighting alongside them at the back of the plateau in the rockslide megabattle. See They're alive! for some chat on that matter. On the other hand, for a playful look at their many shortcomings see Marines aren't that bright on my 'Annoyances' page.
Draw the enemy fire away
On Heroic or higher, Marines are likely to be quickly massacred by the Covenant if you leave them unsupported. When supporting them, you can help them survive all the longer by drawing more of the enemy fire towards yourself, if you can afford to take the punishment. Sometimes I'll even jump into a stream of needles heading for a nearby Marine who'd otherwise be done for; they're useless at dodging. Now that's taking it seriously! And does he even thank me? Hell no.
Fumbled grenades
Marines can be pretty effective with grenades, but have a bad habit of dropping them or fluffing the throw after being disturbed by incoming fire. As such, it's highly risky to be standing right among them. Safer to keep some distance, which is also better for being able to move around freely. Sometimes you might be well ahead of them, but a fluffed throw can easily head your way. With practice however, you can get sensitive to the sound of a frag grenade landing nearby in the heat of battle, and instantly head away from it.
Snipers
Snipers are way more effective than normal Marines and can wipe out plenty of the enemy for you. Unfortunately their dangerously indiscriminate fire also tends to wipe out members of their own squad, or nearby cyborgs! As such, they're a mixed blessing. Stay well clear of their line of fire.
Forgive and forget
Whether by accident or intent, when you kill a few Marines (gunning down three will usually do it), the rest turn on you. However, all is not lost. If you wait long enough, they'll forgive you, uttering warnings such as "Don't try that again!". See Forgiven cyborg for an investigation of this forgiving behaviour, which I personally knew nothing about for ages until finally experiencing it one day.
Marine resilience
Marines are usually armoured, but in level 2 there are lots without armour, presumably because they had to get off the ship in quite a hurry! Armoured Marines are usually much more resilient, which makes sense of course. It was a long time before I realized this, prompted by a spot of email from fellow cyborg O'Reily. Based on some experiments that involved seeing how many AR rounds it took to kill a perfectly healthy Marine, armoured Marines are at least twice as resilient. It's hard to be any more precise because there seems to be some randomization built in. For example, on Heroic a perfectly healthy armoured Marine typically takes from 12 to 16 rounds to kill, and an unarmoured Marine from 4 to 6 rounds. There can be exceptions though: in level 6 I met an unarmoured Marine wearing a jungle hat, who took 13 rounds to kill.
Incidentally, resilience also varies between difficulty levels. If the armoured guys at the start of levels 3 and 4 are anything to go by, Marines become more resilient with difficulty level, except that Legendary may be the same as Heroic. On Easy I found that they took 6 to 9 AR rounds to kill, on Normal 10 to 12, on Heroic 12 to 16, and on Legendary 12 to 17. Of course, this increase in Marine resilience is somewhat outstripped by the increase in enemy aggression. The higher the difficulty level the quicker Marines get wiped out, as you've doubtless noticed!