One simple thing that I've found to help me write comments of all types in a positive and unoffensive manner is to simply proof read the comment before posting. I re-read all of my comments aloud in my head, and make necessary adjustments before hitting the submit button.
I find that emotion and enthusiasm can lead to gratuitous negativity without even realizing it, as those things don't always translate well to plain text. Aside from posts, this applies to emails and texts.
Since I started this, I am almost never accused of coming across in a negative tone, even when I want to convey disappointment or other negative emotions. There's usually always a way to craft it so that it's perceived as constructive criticisms, and thus, well received.
Write with emotion, and proof read without.
As a bonus, I rarely make spelling or grammatical errors, except occasionally out of ignorance.