Iceman's powers have evolved considerably over the years. Originally he had a normal human and an ice-form. In normal human form, he had the strength, agility and durability of a typical athletic male of his size. In his ice-form, his strength and durability was enhanced.
According to the laws of thermodynamics, cold is defined as the absence of heat, and therefore there is actually no such thing as "cold." Therefore, Iceman cannot emanate cold, but rather must absorb or dispel heat. Since heat is energy, Iceman's power then becomes a psionic ability to manipulate energy.
Iceman is able to lower his body temperature without harm to himself, reaching -105F within a few tenths of a second. He is able to freeze any moisture in the air around him into unusually hard ice to form ice-slides and various projectiles and shields. He is also able to summon moisture from the air and create a flood. He did this against the Juggernaut.
Iceman is immune to sub-zero temperatures. Iceman has thermal vision: the ability to detect objects visually by how much heat the objects generate.
Over the years, his mutant physiology has gone through radical changes. First, he was able to fashion an armor of densely packed snow around his human form when lowering his temperature; later, this became armor of solid ice. Eventually, he was actually able to transform his body into organic ice in which his strength and endurance were augmented to enhanced levels, but when he turned his body back to normal, he would be an ordinary human.
For a time, Iceman could no longer revert to a human form as part of a secondary mutation. He was constantly in a translucent organic ice-form. Iceman is also able to reconstitute his ice-form if any part of it is damaged or even if it is completely shattered, without permanent harm to himself. He is able to temporarily add the mass of a body of water to his own to increase his mass, size and physical power. His strength and durability in his ice-form are enhanced beyond normal human levels. He can survive not only as sentient ice but as sentient water and vapor. He has the capacity to manipulate all forms of moisture. He has all of the abilities to generate projectiles, slides and shields that he always has had but they have been augmented greatly. Under the mind control of Emma Frost he was able to deposit his bodily mass into a river and reconstitute his entire mass a great distance away in a matter of minutes.
Writers have implied that Iceman's lack of personal initiative has prevented him from discovering the deeper facets of his abilities and has led him instead to focus only on what might be the easiest manifestation of his abilities, the creation of ice.
In House of M #8, it appeared that Iceman was among the many thousands of mutants who lost their powers due to the magic of the Scarlet Witch, having returned to his normal human form and noticeably sweating. However, at the end of X-Men #178, while repelling an attack of anti-mutant protesters, Iceman's powers reactivated. In X-Men #179, it was revealed Iceman didn't lose his powers, but had subconsicously shut them down out of fear due to the aftermath of the House of M.