A keychain is easy to dismiss because it is small. That is precisely why it needs discipline. A weak one becomes clutter. A good one gives order to something handled every day.
Begin with the hardware
Look first at the ring, clasp, hinge, and connection points. They should open cleanly, close securely, and feel proportionate to the keys or bag they will carry. Excess weight is not a virtue. Neither is decorative hardware that catches on pockets or lining.
Material matters at this scale
Leather should have finished edges and enough body to resist curling. Metal surfaces should feel deliberate rather than thin or sharp. With frequent use, both leather and metal will mark. The question is whether those marks become natural wear or immediate deterioration.
Keep the role clear
For keys, choose compactness and secure closure. Used on a bag, scale becomes more important: the keychain should relate to the bag rather than compete with it. Logos, colour, and ornament work best when the rest of the composition is quiet.
THE GRAY CRAB view: a small accessory should signal intention, not effort. Buy for function first, then material and finish.