The whole rifle.
It's polymer but it's super heavy, threads are inverted (as said above), the receiver is proprietary so you cannot fit aftermarket gearbox shells (and in no time you wish you had bought a different gun). Pistol grip fucks the motor angle. God forbid you want to install some "octarms" parts, wildly off spec even for Ares stuff..
Internals are abysmal, zero air seal to begin with, gearbox is proprietary and poorly made (very thin at the front, where it usually cracks), bushings are made of aluminium (yes, aluminium, they last very little), compression set is bad, especially the piston, it loves to crack at the rear, gears aren't good and due to how the etu controls the cycle (via a magnet) you have very limited options for upgrades, ETU is trash tier, proprietary to the gearbox and has the tendency to fry itself plus if you set battery protection to 11.1v it won't recognise 7.4v batteries anymore, nozzle and tappet plate are bad, plate will probably snap the fin within 2 months of use.
Hop rubber is paper thin, doesn't lift anything past a 0.2 and it will most likely rip apart within 3 mags.
Sauce: i had two Ares M4s.
Edit: I'm sorry, but it had to be done, probably if you asked before buying we could have warned you.