What gives me karma here might not give me karma at other sites and vice versa. A "karma is hard - let's go shopping" comment would get a high rating on reddit

What gives me karma here might not give me karma at other sites and vice versa. A "karma is hard - let's go shopping" comment would get a high rating on reddit and a negative rating here. So your karma is tied to the site, and tells very little about how "popular" you would be elsewhere on the net.