According to TTMIK, saying 이것은 is actually quite textbook and unnatural. Using 이거는 or 이건 like how a native speaker would sounds much more fluent and comfortable.
I wrote 이것이 완전히 달라요 and got it wrong. Is this because ~은/는 may indicate some sort of comparison with another thing and that is the case with the sentence?
"이것이 완전히 달라요" is grammatically valid. However, in practice, a speaker would prefer to use the topic marker since it reinforces the comparison they are making.