Two factors... Clamping Time and Energy Dissapation.

In other words, how quick can it clamp down the incoming surge (miliseconds) and how big a surge can it deal with (joules). Most residential products still use MOVs I believe; often stacked one after another. These work pretty well if designed right.

If you can't find the two specs I mentioned, then brand can be a good indicator. Panamax and APC are two of the best for home products. Not the cheapest, but if you are concerned about good protection those are it. Second tier but still good are products like MonsterCable, Belden, etc. Stay away from cheap generics. The really bad ones are no better than a fuse inside! (SUPER slow)

One last tip... don't forget that often lightning strikes can come in through cable lines/modem wires/etc. Not sure what you are protecting but if it has another wire out to the world, you might want to consider a product that protects that line as well if lightning is your worry.

Hope this helps,