First thing to decide?
What model will you pick?
After that I like to think about materials.
How thin or thick? How long? Size and what my pens look like next to more familiar brands.
Not least important is the writing tip. Nibs.
Section Shapes – How do you like to grip your pen?
Filling – Converter, Button, Bulb, Piston.
Steps? – That little shoulder on the barrel is called a step. Will it fit your hand?
Clips & Roll Stops – Steel and custom options in silver and gold.
Rings and Bands – want to add some spice to your design?
Ink Windows – Do you want to see how much ink is left?
Vent Hole in Cap? – Allow the pressure in the cap to be regulated.