If you remove negatives or iffy comments, the only people who'll want to read are those who whole-heartedly agree with you. You'll drive away any audience who has questions or doubts.
I delete spam--link-filled comments that don't show any sign of having read the post. I screen actively hateful comments, and anything that reveals private data. Other than that, almost anything goes; in over 8 years of blogging at Livejournal and Dreamwidth, I've banned about three people and frozen less than half a dozen threads.
Caveat: My blog, or journal, is personal, not commercial. The important aspect of the comments is "I'm enjoying them;" not that they be on-topic or within a certain emotional range.