Kritikovati is to tell some person that he/she did something which is not so correct or good. Zamerati is when we find out something incorrect in someone´s doing but we do not have to tell it to that person. At example: On je kritikovao Mariju. - which means that he told her what she did wrong and fact that he did not like it Zamerao joj je to što ne ume da kuva. which means that he did not like the fact that she does not know to cook, but it does not mean that he told it to her.

The difference is in the feelings of the person who complains. "Kritikovati" is to criticize, like in English: to tell a person what they're doing wrong. "Zamerati" implies feeling offended, being angry with the person because of something they did. It means to object because we're offended. For example: "Zameram ti što me nisi pozvao" - we're criticizing and we're saying that the act offended us. If you "zameraš" you can say it or keep it for yourself. It has nothing to do with that. Usually you say it, at least in the form of badmouthing the person in question. :)