Superman is vulnerable to Kryptonite and Magic based attacks.
Superman is vulnerable to the Black Mercy, which Mongul used to trap him in a catatonic state.
Superman's usual calm can be shattered, as he has developed a murderous vendetta against Darkseid - to the point he was willing to destroy himself rather than let Darkseid escape (prevented by Batman) : he has also experienced a mental breakdown in regards to Mongul, who he nearly beat to death but relented - which allowed Mongul to attack in turn.
Superman is usually kind-hearted, trusting and merciful - traits that can be exploited, manipulated or even cause him psychological stress : he has stated that he often holds back his power as a result.
