Edit: As of August 2016, it is no longer required to have a Google Plus Profile to leave a comment in Google Play Store.
However, it would have great sacrifices to comment anonymously in Google Play Store as you have to remove your Google+ Profile and as long as you create one again, all your 'previously anonymous' comments will immediately be linked back to your new G+ profile.
Note: The following text is obsolete. Refer to text above.
Due to Google's requirement since November 2012, a Google+ profile is required to leave comments under contents from Google Play Store, as is discussed here. All those comments left by anonymous users without a Google+ Profile before Nov, '12.