Honestly I think it should be L56+. Until you've unlocked all the maps, you're not actually exposed enough to what's out there (which has a serious impact on how you rate things- restaurants, where variety is high, are often rated more harshly than barbershops, where you really just pick one and stick with it => low exposure means high ratings).
If you're a L5 and you've just played Medium Earth for the first time, you're probably not even aware of MME and can't really properly rate it without having a good context for what deserves a 1 and what deserves a 5. At that point you're likely just going to give it your anchor rating.
Simply rating or reviewing a map doesn't require much Warlight skill.
Last I checked, getting to L30 doesn't require much skill either. I mean, you're L55 and clearly didn't get there by having a lot of skill. :P
Instead of levels, though, we could go by games played (100+?) or days since joining (50+?). Those would avoid the negative consequences of a level restriction.
Edited 5/27/2016 22:55:02