What in your opinion defines a good roleplayer?
I think what defines a good roleplayer, in my experience, is the ability to clearly, and concisely describe your actions so that everybody can definitively understand your actions. I also define a good roleplayer as somebody who is patient, and takes realistic approaches towards things. Understanding not only your character, and your character's motivations, but understanding how society sees your character, and what roles your character is expected to bend towards based on ethnic and cultural levels, as well as gender roles in society as well.
What qualities must a good roleplayer have?
Patience, more than anything a good roleplay needs patience to let scenes unfold, and occur in front of them. They also need to able to look at larger pictures, and look beyond what will happen tomorrow, but how certain things will affect them weeks or months down the line.
What do you look for the most in someone when you roleplay with them?
Do good roleplayers necessarily have to have knowledge in multiple types of roleplay (government, legal, illegal)?
Absolutely not, it wouldn't make sense for most characters to know everything about other types of roleplay, understanding certain gangs or government divisions etc.