Grundy has also proven to be able to absorb certain forms of energy and manipulate them at will. This ability has always been subconscious to the point of being accidental, though, but it would suggest that Grundy is even more powerful than he has the intelligence to realize.
The weird elemental energy that imbues his form with pseudo-life makes him virtually indestructible - he is impervious to bullets, fire, and extreme cold, and can survive indefinitely without food, water, or oxygen.  The only thing that has ever been able to hurt him was the Mist's special toxins, developed to take out Grundy's plant tissue body.
Even when he is destroyed, he has always been reborn sooner or later. The soul of Cyrus Gold enjoys his pseudo-life through Grundy, even though Gold's whole personality is never present in Grundy's "life." Grundy's personality differs from incarnation to incarnation, but he seems to retain some memories of his experiences from one time to the next.
Grundy can perform physically strenuous tasks for an indeterminate period of time before tiring.
Solomon Grundy possesses tremendous strength and stamina, the level of which varies with his different incarnations. Sometimes, he can be held by a group of normal humans. On other occasions, even Superman has not been able to defeat him.