What's Driver Unit Size got to do with it?
The bigger the better with respect to loudness of sound. Simply put: the bigger the driver unit, the larger the speaker, more powerful is the output. So basically a 10 mm driver unit will be preferable to a pair of earphones with 6 mm drivers.
What about Frequency Response?
What's funny is that although humans can only hear a 20-20 frequency range, it is thought that the greater the frequency range offered by a set of headphones the better they will sound. So basically a frequency range of 6-23 is wider and thus better than a pair of earphones with a 10-20 range.
And what about Impedance?
The lower the headphones' electrical impedance, the easier it is to get a higher volume. Simply put: lower the impedance, lesser the drainage of power from the battery of the portable device. So basically, headphones with an impedance of 24 is better than one with an impedance of 32.
And what does it matter if a Plug Type is Gold or Nickel plated?
This all boils down to durability. Headphones with a nickel plated plug won't last as long as one which is gold plated. But then nickel also costs lesser than gold. So basically, if you want your headphones to last a long time, make sure you go for one with a gold plated plug.
This is just an overview to help you decide without going into the technical details too much but if you really want to really educate yourself do check out our Technical Guide. After which you can also rightfully call yourself a Headphone Expert!