A picture really is worth a thousand words. This one, for instance, represents the Martian Manhunter’s time on the Justice League.
Based on what they have to say about him, I’m going to presume that (once again) J’onn took notes on his fellow heroes, discovered their secrets, and this League was far less understanding than the one in JLA Year One.
But we, the readers, know J’onn is a member of Stormwatch now, saving everyone from threats before the Justice League even becomes aware of them. He’s fighting for the big picture, not for the camera. And apparently he knows something is coming.