A is for Alpha Centauri.
B is for Betazoid.
C is for Cardassia III.
D is for Denobulus.
E is for Earth, capital and central planet of the Federation.
F is for Ferenginarn.
G is for Gorn eyes. Some had silvery compound ones, others had more reptillian ones with irises and pupils.
H is for Holographic testicles.
I is for Irradiated. If you stood too close to an unshielded warp reactor, this could happen to you.