i guess ultimately, i'd have to say it's a profession, a career choice. but that i feel that it's probably similar to any career choice and what i mean by that is that some people know from a very young age that yes, this is for sure what i want to do with my life and some people come into whatever field at for different reasons, later in life.
having said that - i don't think that signifies if that will make the T a "good/great" one or not.
(i'm going to state now that i have seen several T's, and this is all just opinion, and that i hope i do not offend anyone. just stating my thoughts
1. fact is, a T cannot probably possibly "click" with EVERY client they see. so although a T may be considered absolutely wonderful by the majority of their clientele, i'm sure there will be a few, or maybe even over the years, just as many, that couldn't click with the T, and felt exactly the opposite.
2. i think, as was touched on already i believe, that what makes a "great" T is someone who can find a way to successfully blend the educational/by the books/what you learn in school stuff - with the things you don't or even can't learn in school/training. by this i mean, personality i guess, for one example. a T has to be able to connect with a client in order for the relationship to work, make the client feel safe and heard and validated, help them, and over time i'm sure care a great deal. but then at the end of the day, when they go home, maybe have a thought or two, but be able to disconnect from their job. i'm sure this is hard.
(do you think T's use some of the same thought-stopping tools they talk about? 
then you have to consider T's who answer after-hour calls and emails. wow. i'm sure it can be hard. this post has opened my eyes to that fact even more, which i think is great.
so i'm sure they definitely have to be committed and want to help.
3. one last thing, because i'm sure this has become painfully long for some - sorry!

anyway, i think it's possible for some T's - along with any other kind of professional - to lose track of why they chose the job they did. to become focused on money(understandable, i suppose, it is their income), filing papers, insurance, etc etc. maybe jaded is the word. in this case, (and maybe because i encountered this semi-recently with my last T, i feel) - i can only hope that the T can reconnect to why they originally started out doing the job.
ultimately, as has been said, a therapist is human in the end. we all, if able, try and choose what job/career we think we'll be good at. some are just better than others, i guess.
i guess that's my very long (again, so sorry!) two cents for now.