What does define the quality of the master thesis?
Any such definition is necessarily broad and somewhat vague. Here's my attempt at offering a categorisation:


An instance of an outstanding master's thesis is one that significantly advances state-of-the-art research, such results are rare.
An instance of an excellent master's thesis is one that contributes to state-of-the-art research, such results are infrequent.
An instance of a good master's thesis is one that satisfies the institution's criterion for the highest grade, such results are common amongst PhD students.


(This categorisation is highly subjective.)