I disagree with a lot of choices actually. In my opinion,

You should add a "situationally solid" tier because a lot of masteries fit into that
I think Havoc is mid tier at best;
I'd move Sunder into top tier offense;
I'd move Havoc and Alacrity down a tier;
Brute Force is situationally solid
Offensive Mastery is great for junglers
I'd move both Tenacity and Nimbleness to top tier defense
Strength of Spirit is way better than you have it. Possibly top tier otherwise second tier for sure.
I'd move Awareness down a tier (though it is good)
Meditation I think is top tier for utility; maybe second tier.
Expanded Mind in the same tier as Meditation is that a joke?
Utility Mastery is great especially on junglers.
I personally think Greed is much better than people give it credit for. For 1 point with no requirements, it's pretty decent I think. I often take it.
I would also move Good Hands up a tier. It is situational because you have to die for it to be useful, but I find it invaluable for when I do die. And if you've ever seen me play Singed, you know that is very often.

One of the main things people seem to be saying is "I use this mastery all the time with my specific build on my specific champ, so it's higher tier". I wasn't clear that when I ranked these, I ranked them for EVERY build of EVERY champ. For example, if you're going to get points in utility, you ARE going to get awareness, regardless of who you use. If you go down that far, there is absolutely no reason not to take quickness. On the other hand, how many people are going to care about creep damage? One out of five assuming every team has a jungler in every game.

Basically if a mastery is higher tier, it means more people use it. That's all.

In that case, Good Hands should be top tier, because who goes utility and doesn't take that?

Did Guinsoo just forget that he could lock threads? Or maybe he doesn't have the power.

I'm still hoping for the mastery tree changes D: