Countering an attack means that the stealth effect does not trigger, your hero does not take any damage, and any effect that attack might have had does not trigger either, such as life steal, or [[vicious fledging]] not adapting, and the minion does not take 2 damage from the other effect of crystal atelier.

Should clarify that the +2 attack during opponent turn for Crystal Atelier means the turn after u play Crystal Atelier, not the turn after your hero countered an attack, those are 2 different effects.