Accuracy and flatness of the shot is all I really look for in a gun review... It's why I own lots of pumps, specifically Carters.
You will want to include features that people these days really look for. From my research that would include the following.
-How laser-like it shoots (if laser is available to compare to).
-Tack/nail driving test to make sure that said gun can drive tacks/nails.
-If it shoots t*** Feature omitted due to inappropriate shooting of body part***
Further research revealed a few more features some may be interested in
-How old/beat up the gun is
-If said gun can bring back memories of playing when the owner was 25 years younger
-If the gun can somehow be made into a pump