I would agree with Muirgheasa's definition except I would add the requirement that constantly "pushing the envelope" and not being afraid to experiment is also a requirement.
(According to this definition a "great brewery" WILL produce some individual beers that aren't that good, and sometimes a beer that is even bordering on bad; this is a part of furthering the art and a requirement to me...)
Hmm.. Even after reading this immediately I can think of exceptions to my own "rule" above... I guess more accurately, I believe that Muirgheasa's definition+ showing a true passion for beer and the brewing art is a requirement; for some brewers this is manifested as constant experimentation, for some this is manifested as an unwavering devotion to quality regardless of cost, for some it's reviving lost/dying beer styles, and for other's its striving to master one beer style or the styles from a particular culture and provide as accurate of a "reproduction" as possible. (Actually, I think "great" breweries probably would show multiple of these traits.) -In a nutshell; obsessive-compulsive disorder (OCD) brewers are the best brewers in my book....
Adam