That's part of the problem here: there is no legal definition of journalist.
A journalist is "someone who does journalism". Traditionally that meant publishing stories in a newspaper, radio or television company, and it was fairly clear-cut who was a journalist and who wasn't. Nowadays not so much - the main question being "is a blogger a journalist?".
Assange was supplying information to, and co-operating with, major media organisations. I think any definition of journalism would have to include the activities of Wikileaks, and therefore Assange is a journalist.
Where the line is drawn is also a good question. Government would like it drawn at the lines laid out in the pertinent secrecy laws. But clearly if just classifying something as "Secret" would then put it out of reach of journalists and make unauthorised knowledge of it a criminal offence, then that's not serving society. All sorts of political misbehaviour would be classified.
And yeah, there are lots of shades of grey. Lots of things we have to decide as a society:
Are governments allowed to keep things secret?
Who should decide what is kept secret?
If someone discovers that a government agent is doing something illegal, but the activity (or agent) is covered by secrecy laws, how do they make sure that the activity is prevented and/or punished?