1. 'It's a great idea.' 2. 'That is a great idea.' 3. 'This is a great idea.' There is a small difference between the sentences. I think the difference is linked to the degree to which the speaker feels involved in the idea. In 1. the speaker is probably less involved - they are giving an objective judgement/ opinion. In 2. The speaker feels more involved. In 3. The implication is that it is, in some way, a shared idea - or an idea about a shared subject/ plan/ strategy/ direction etc...