The way it sews mainly @chrisash. Some stuff is a dream, some stuff you have to pay a bit more attention to things like how easy it pulls through leather, the finish it holds, strength.
For instance, I like the Chinese knock-off Tiger thread. It behaves well for me and the strength is fine. But Tiger is a bit more waxed, or waxed with a different recipe or something, and it is a bit stiff and make it a wee bit easier for me to use. Given the availability and cost of Tiger though, I opt to sacrifice a little bit of ease of use on my part for the cost. They are very similar in my estimation. Also, I have noticed the Tiger thread seems to "scar" a bit compared to the knock-off. When you are sewing and you stab through the thread while coming through the whole, I can see it more when using Tiger than with knock-off Tiger.
Maybe its just me but those are a few things I've noticed.