1. Face-concealing masks, as popularised by Killzone
's Helghast and Halo
's Master Chief. A way for 'busy' designers to get around rendering faces, or to have more enemies on-screen at once - as a flat mask uses way less polygons than a bumpy face.
2. Sometimes they've got glowing bits attached to them - see Haze's Mantel soldiers' arms and the TimeShift guy's chest.
3. Big, fat, stupid guns that weigh more than a child and have loads of extraneous 'bits' attached - to remind you they're futuristic.
4. Futuristic body armour - sometimes, oddly, a bright colour to make it 'stand out', thus attracting loads of attention from your enemies. Almost always equipped with enormous shoulder pads and mechanical codpieces. The armour is purely aesthetic - it protects the wearer for about two shot's worth of fire max.
5. Batman-inspired moulded six packs. Amazingly common, yet stunningly pointless. Used to boost a character's macho-ness. Designers reckon six-packs mean a man has cojones.
6. Nowhere to keep guns - body-hugging armour is designed specifically to accentuate every rippling muscle, you see. Adding lumpy gun holsters would detract from all that intensive rendering on the body model. Bah.