Outside of construction flaws, I dislike folders that are difficult to open one-handed. This is generally due to a combination of:
1) Opening device (thumbstud, disk, etc) that is hard to access (e.g., nearly flush with the handle) or is too small or polished and rounded.
2) Location of the thumbstud with regard to the pivot - i.e., the closer the thumbstud is to the pivot, and the narrower the handle, the more specific does the force to exercize in order to open the knife. If the thumbstud is well located, the fulcrum is optimal.