You can't really put it down to one thing as everything works in concert to make up the final sound of a capsule.
In particular the diaphragm material thickness,tension/tuning frequency and the back-plate holes and chambers affect tonal,transient and polar pattern characteristics . Also capacitance, which is determined by the distance between the diaphragm and backplate affects output and proximity effect, the smaller the gap the higher the capacitance and the greater the output and proximity effect, it's a but like bringing a guitar pickup closer to the strings on an electric guitar. Another factor is the polarization voltage higher voltages increase output but reduce the ability of the diaphragm to move freely,so the polarisation voltage is also critical.
If you compare with an electronic circuit, it's not the transistors, caps, resistors,... that count, it's how they're put together, in short it's the design.
There are endless debates about diaphragm thickness, PVC vs. mylar, Al, Ti, Ni vs. Au, they all give slightly different results, but tuning is probably the most important single factor, and all the other less important are just different flavours of equally good designs. Different applications justify different flavours.
All of these factors, you can think of, can affect the capsule. Material, thickness, gold-plated thickness, gold-plated area, purity of gold, resonance frequency of film, internal stress of diaphragm, design of backplane, thickness of gap gasket, these are the factors that affect the capsule