Obviously, the commenter did not read the IDF report, but it brings up an important point that others probably missed as well: The IDF interviews as many Palestinian Arabs as possible, and nothing (outside of Hamas) is stopping Gaza civilians from filing complaints or testifying.
As the second Cast Lead report mentions, quoted in Ban Ki-Moon's report:
The MPCID has sought assistance from non-governmental organisations (such as B’Tselem) to help locate Palestinian complainants and witnesses and to coordinate their arrival at the Erez crossing point to Gaza, to allow interviews and questioning. To date, MPCID investigators have taken testimony from almost 100 Palestinian complainants and witnesses.
It also goes over the difficulties of getting evidence in a situation like this:
The unique difficulties involved in the investigation of alleged violations of the Law of Armed Conflict in the battlefield should not be ignored. They include: the inability to secure the scene for forensic and physical evidence, either during a battle or after, when the territory is under enemy control; the possible destruction of evidence during fighting and the possible manipulation of the scene by the enemy; the need to recall reserve soldiers back for questioning; the difficulty of accurately identifying the location of an incident, when it is described in local and unofficial terms and slang; and the need to locate the adversary’s civilians as witnesses and overcome their natural suspicion and fear of reprisals by their authorities.Also,
47. Information on alleged misconduct of soldiers reaches the IDF authorities in various ways, including:
o formal or informal complaints by alleged victims themselves or family members;
o complaints by commanders or soldiers who witnessed an incident;
o reports by non-governmental organisations and the news media;
o complaints or letters by non-governmental organisations, journalists, embassies, or international bodies; and
o complaints forwarded to or filed directly with the Military Advocate General’s Corps by the Israeli Police and other law enforcement agencies.
48. Any person may file a complaint with the Military Police at any civilian police station regarding alleged misconduct by IDF soldiers. Gaza residents can file complaints directly in writing (in Hebrew, Arabic, and English), through a non-governmental organisation acting on their behalf, or through the Military Liaison that works directly with the Palestinian civilian population.
In other words, the IDF investigation does everything humanly possible to discover and investigate complains as thoroughly as possible. They will work together with NGOs - including NGOs that are openly hostile to them - to arrive at the truth.
Goldstone did not take anything close to the same effort to look at or consider Israel's side of the story.
Another telling part of the report, in light of Hamas' denial of apologizing for any Israeli victims of its aggression, is this sentence (para 89):
Israel’s efforts to comply with the Law of Armed Conflict do not lessen its regret for the loss of innocent lives and damage to civilian property.