First of all, guys, he's not Captain Marvel anymore -- He's Shazam!
Secondly, Captain Marvel/Shazam is not more popular than Aquaman. Maybe back in the 40's and 50's he was, but now, I'd say they are about even in the popularity department.
My one concern for introducing Shazam so early in this new movie line (and make no mistake, I'm pretty sure we'll be following up this movie with more New 52 era stories) is that if this is the new introduction and origin of the Justice League, it kind of diminishes Superman's intro here by having someone similar and almost equally as powerful as Shazam introduced within the same movie. I think they should have held out on giving us Shazam, Aquaman and Martian Manhunter and instead just stuck to Superman, Batman, Wonder Woman, Green Lantern and The Flash (and I guess Cyborg, too) for this movie -- but I'm sure some fans are impatient and tired of having those same characters, so they need to throw in one more to change it up (then again, I thought that's why we have Cyborg now).