In the first sentence, saying, "Don't you think" suggest that you want their agreement and are therefore not as sure. The second sentence using "Wasn't it" is usually expressed by native speakers with more enthusiasm and confidence.

Thank you for your comments.

In this case, they are quite similar, although, 'wasn't it' is slightly more casual/familiar, maybe. It may also be regional to some degree. For 'wasn't it,' it can also be used just to confirm something factual/objective rather than asking someone's opinion ('don't you think?' Is more restricted to when asking someone's opinion, thoughts, etc). 'John, wasn't it?' For example, if you met someone previously and are confirming that their name was John. 'It was on Tuesday, wasn't it?' 'Wasn't it' can also be used to express uncertainty and/or can be used more rhetorically, i.e. you're not really looking for an answer. This is less the case with 'don't you think.'

とても有意義なセッションでしたね。 "It was a very informative session, don't you think?" ✅ あなたはそのセッションはとても有益だったと言っているのに、他の人の意見を求めています。 "It was a very informative session, wasn't it?" ❌ 文法的に間違っています

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