Exploits and people who utilise overkill (2 guys with rocket launchers vs. one fellow with a pistol?...that can't be fair), if a guy kills me in a genuinely skilled way (eg: strafes around me with the shotty, uses melee or uses the sinper in a nomadic fashion rather than camping in one location or attempting to no-scope with the damn thing) then I have no problem with that and will useually congratulate the person in question, otherwise, its useually the other guy making himself out to be Zues himself while repeatedly calling me a 'noob' or 'pussy' (at these times I don't have to say anything).
Hackers, I've only encountered this once or twice but its still annoying all the same. I was playing CnC 3 online as GDI once and right at the begining of the battle (when most people are bulding their power plants,refineries etc.) a NOD barracks (remember that I was GDI) pops up in the middle of my base with a constant stream of engineers capturing and selling my buildings while the player responsible is continually howling with laughter over the mic and generally hurling abuse.