The missions who weren't played at least 5 times don't give any credit for playing them. Check "review content", you'll see no mission is eligible there.
Hmm I can see it from both points of view.
On Cryptic's side:
Before people play it the game has no idea how long it takes and whether it would count. At the same time if it lets it through everything before the 5 reviews then it could easily be exploited. People can play it over and over without reviewing so it never leaves the review period.
For the Author's side:
Pretty simple, like you said how do we get the first 5 reviews?
Is there a safe middle ground? Or will we have to reply on people who go out of their way to play missions that do not reward them.