The ISS has three (four?) types of docking ports. The Russian side has one standard with two types (active/passive?) and the US side has either a PMA (soon to be updated with the IDA to the NDS standard) or a CBM port.
The PMA (Pressurized Mating Adapter) with the IDA (International Docking Adapter) allow for vehicles to dock and undock without intervention and thus are designed/designated for crew vehicles. The IDA brings the PMA to the NDS (Nasa Docking Standard) that CST-100, Dragon V2, and possibly Dream Chaser cargo will match.
The CBM (Common Berthing Module) is meant for attaching larger modules like the various ISS Modules (Unity, Harmony, Tranquility, Columbus, Kibo, PLM, Quest Airlock, Cupula, BEAM, etc). Thus it is not really meant for unattended operation.
Thus a critique of using a Dragon (cargo/v1 whatever you wish to call it) for crew, even if all other concerns were met, is that it cannot unberth on its own in case of an emergency.
So what exactly keeps it from unberthing on it's own?