Comfort and grip: Check this before you buy. If the secateurs come in sealed packaging ask to have it opened. Check the handles to make sure they will be comfortable to grip for extended periods. You should be able to open and close them easily without over-stretching your hand.
The leverage you'll get depends on the length of the handles. But if they fit your hand poorly, open too wide, or if the spring is too powerful, your hand will quickly tire and you may get blisters.
Safety catch: The catches for locking secateurs in the closed position vary widely and are probably the greatest cause of irritation for users. The most common catch is a simple thumb-operated lever on one side. However, a lever catch that is loose or sticks out from the handle too far can accidentally engage during use.
Others have a sliding catch, usually operable from one side. On some models the catch can be worked from either side or from on top of the handle, and can be used easily by left or right-handers.
Check that the locking catch on your preferred model can be worked easily with the thumb of whichever hand you use. It should work smoothly without sticking, and you shouldn't have to grip the handles tightly closed to get it to engage.
Pivot adjustment: A real frustration for secateur users is a pivot that keeps coming loose. Look for one that can be tightened. The best types are those with a separate locking tab (with teeth) that positively fixes the adjusting nut in place. Others have locking nuts, or screws that can loosen over time.
Ratchet mechanism Some anvil secateurs have a ratchet-type mechanism that reduces the effort required - you squeeze the handle several times to make a cut.
Visibility Bright colours make it easy to find a dropped pair of secateurs. A wrist strap also helps keep hold of them.