In most cases, the following features are dealbreakers for folders for me (but there are some exceptions):
-bearings (especially when they run directly on titanium or Aluminium)
-weak detent
-unreliable lock/lock-up
-button locks
-assisted opening
-short blades (<3 inches)
-long blades (>4.5 inches)
-sheepfoot blades
-extreme thin blade stock
-short handle; for my hands I want 4.5/4.75 inches or more on a folder, depending on design and shape
-low traction on handle
-handle not covering the blade with some extra space
-handle materials like wood, mother of pearl, Lapislazuli and so on
-carelessly designed/made clip
-no finger guard (at least some slight curvature protecting my index finger)
-materials and workmanship not up to the price point
-price point under ~50 USD or above ~600 USD
For fixed blades:
-short handle; for my hands I want 5 inches or more on a fixed, depending on design and shape
-ridiculous designs and shapes (very subjective, I know)
-blade length under 4 inches or over 7.5 inches
-no full tang
-saw on spine
-carelessly designed/made sheaths
-scales made from metal, leather, wood,...