I'd say it's brave, not heroic.
Brave just implies he has guts - which is true. Heroic implies both that the action was brave and challenging, and that it was undoubtedly in the right and had an overall beneficial effect. I'm really not sure that is or was the case.
Essentially, there are some bad reasons why conversations are kept secret - for example, because nations are torturing people or planning illegal wars or cynically manipulating situations to their own gain. In these cases, it is clearly a good thing when someone exposes it. On the other hand, diplomacy played with all the cards showing is a much more dangerous game. It allows countries to find pretexts for war much more easily, it could include transmissions of weaponry blueprints or troop movements that could put innocent people in danger (not just the troops, eg "Ooh, this cable shows us our enemies are planning to move all their troops to this area. We can then bomb the civilian towns over that way without repercussions"). It's also important that people are able to speak their mind when planning policy, so that governments can hear a range of opinions and get a good outcome. If political scientists or policy experts know their views will be leaked they will be less likely to say anything at all, weakening policy and giving worse outcomes. It makes diplomatic solutions harder to achieve to problems as well as politicians of different countries cannot make difficult deals without everyone knowing, potentially increasing the risk of problems being solved via military means.
The problem I have with Assange is that he doesn't distinguish - he just sees the first sentence of the previous paragraph, decides secrecy is evil, and then releases everything. It's bravery, I'll give him that, but it's not heroism.