'it' is probably used more often, more generally. I'm sure if you say 'this' you would be understood, it's just that 'it' might sound more natural. There are also cases where 'this' would be inappropriate. For example, I might ask if you like a particular movie. You should say 'i hate it' rather than 'i hate this'. I have similar problems in french with ce, il, ça etc.

Both "it" and "this" are pronouns, but "it" in this case is an "object pronoun" and "this" is a "demonstrative pronoun." So what's the difference? Demonstrative pronouns are used to "point to" or "indicate" something that is close by. When you say "I hate this" you could say "I hate this _______." For example, "I hate this situation." You are pointing to the current situation, or the situation just talked about. You are just leaving out the word "situation" because that word is already understood by the context of the conversation.

" I hate it" refers to any "thing" or "activity" that you may be talking about. "I hate this" refers to something you are doing or something you may be pointing to at the time. In general terms, when you say, "I hate this", it implies something current or present.