Is it appealing?
I prefer this question to is it interesting. Synnibar was interesting, but did dragons attacking mutant-driven hover bikes with mages on the backseat really have much appeal in a general sense?
Is it original (or needed)?
Is this really a new concept? If it is, is it one that is needed to make actual game play better? Let's take a simple game of darts in a roleplaying sense. Some folks would simply say roll a d20 and whoever roles higher wins the throw. Some would add a Dex modifier. Some might decide a series of roles with a Dexterity modifier, bonuses or penalties for level of intoxication, size bonus (small characters receive a penalty), a percentile check for the drafty room blowing the dart slightly off course, and a concentration check to avoid looking at the pretty barmaid are very important to the realism of the game. But, and this relates to question one, is that level of detail needed, and is it broadly appealing (assuming the point of designing a game is to garner as large an audience as possible and not just provide a game for a select few, which is an entirely different question of game design!)
Is it fun?
Subjective, yes, but with feedback and looking at sales and comparing it to other games or previous editions of the same game, one can get an idea.