good point. two reponses:
you can assume that anyone who is rated exactly 1200 is NEW :) additionally, you can look at their stats and see their Glicko RD. if it is really high (i believe they start at 350) then you also know they are new. you can look at somebody's stats before you begin a game, and can change the rating range you wish to play in your preferences.
finally, if you lose to somebody with a high RD then your ratings should only change very very slightly - that is the point of the Glicko RD.
Feature Request:
It's kind of a drag playing someone rated "1200" who turns out to be an 1900 level player who just joined chess.com. When I lose, I lose points as if I had lost to a real 1200 player. So, it would be great if there were a preferences setting that could allow me to avoid challenges from people who had played less than a certain number of games. That way, I can risk my rating to a real 1200 player and if I lose, I deserve to lose those points.
It's also relevant to people who would rather play folks who've established a track record.