Grey already touched on a few of the reasons that Stormwatch #6 was an important issue to the DC Relaunch universe but there were two more things I wanted to point out.
A few months ago, I suggested that Demon Knights #4 was a very important issue as well because it seemed to hint at the origins of Captain Marvel. This was followed up in Stormwatch #5, where once again it looked like we were seeing the Captain Marvel logo.
In Stormwatch #6, this logo comes up again, more clearly showing itself on a map of hidden treasure and weapon caches that the former leader of Stormwatch had hidden on Earth:
Eagle-eyed readers might notice that the Shazam logo is actually backwards on this map, but remember that Stormwatch is looking at this from the other side. Which makes the black logos backwards for some reason.
BUT! There was also another major detail that popped up in this issue. Martian Manhunter has never been part of the Justice League!
Of all the changes for DC Relaunch, this has been the biggest surprise for me!