[TenMoonsNorth]

What makes Balanced Armatures sound "thin", makes electrostats have "effortless treble" and orthodynamics have "bass slam"?

 

[MaciekN]

In all three cases the general signature is a consequence of their design principles.
 
Balanced armatures are by design very effective over a relatively narrow range of frequencsies, which makes them rolled off at both extremes with articulate midrange. Of course this signature would apply mainly to single driver BA headphones, with more drivers you cover more bands.
 
Electrostats have effortless treble thanks to a very thin film diaphram suspended in an electrostatic field, unlike orthos or dynamics that have a voice coil attached to or printed on the diaphram. Ligher diaphram means that it can more "effortlessly" start and stop, even at very high frequencies, giving a very detailed sound.
 
With orthos if have strong enough magnets and a good seal from pads then you get a deep and clear bass by design, as in the previous examples this results from the way the diaphram is moved and suspended.
 
By comparison the strenghts of traditional dynamic drivers lies in that they are cheap, less complex in design, relatively light, quite effective and cover a reasonably wide range of frequencies with one driver.
 
There is more to it than just one sentence about each of these designs and if you are really interested in this you would do good by checking wikipedia and a few other sites to fully understand it.
 
Hope this helps to get you started.