All criticism (positive or negative) of any sort of art has an implicit "I think" or "I feel" at the beginning. Adding or removing the words doesn't magically change the criticism so it is suddenly more or less passionate.
I'd argue that this isn't always the case. When you omit an "I think", people may infer that what you state isn't just your personal opinion, but a feeling other people may share, a more
objective judgment on the matter (possibly from someone who knows what s/he's talking about); if you think your opinion has a more general value (you're an expert on the subject, your opinion has garnered consensus in other instances etc.), you can safely omit the "I think", but when you're just voicing your personal feelings regardless of what other people might think,
I think you should make it clear.
It's another matter if you don't care about these small details, but as we're different people with different cultures, scattered all over the world; it's not easy to tell when someone's being rude while communicating with someone else, especially when writing instead of talking. Of course this goes both ways: a little bit of tolerance is necessary on the receiving part as well, but that's usually the case only with complete strangers.