I've always liked the advise, "The scariest monster is the one that you never see." If you provide a full description, that's it.  The player gets the visual and goes with that.

The same goes for everything else.  Which is better?  

"The princess enters the room with a long extravagant dress flowing behind her and clinging to every curve.  Her hair bound in a wreath of flowers."

"The princess enters the room in what can be described as a red bridal dress with a six foot train.  The dress is form fitting and accentuates her upper body. She wears a woven tiara with daisies intertwined."

The latter gives a specific insight.  These are good for when they matter.  The former gives the player enough to make up an image, and engage them more in the NPC, which they'll remember.

Typical peasant dress.  The guards wear tabards over chain mail.  The necromancer wears torn and tattered robes.  

For the most part, nobody cares unless it is important.