One specific instance was when I submitted an XKCD shortly after joining. I eventually realized it wasn't really what people here were looking for. When elections rolled around, I submitted a Wapo editorial that was really off-base, and the reaction I got in response was deservedly negative.
However, in this process of learning to be a more civil and well-reasoned person (which I do credit a lot of to the good folks here on HN), I've always found a measure of grace. I don't think the reactions I've gotten for anything I've submitted or commented with have ever been really mean. Criticism has always been positive. This is the main aspect of HN, besides being intellectually stimulating, that I've come to appreciate most. It has really helped me to grow (I'm a college sophomore now and I really am grateful to the kind people here who've taught me so much since high school).
I agree with pg's general observation that things seem to have become a bit meaner and less constructive. I'm not sure how to fix the front page, but the type of comments I find representative of the old HN are informative, well-reasoned, and forgiving comments. I entirely agree with your bit about "I disagree" comments, though I don't think it would be as much of a problem if it were done a little more nicely. It's okay to disagree with an article, but the tone that some people take, mehh. That said, I almost always read the comments prior to reading the article, because I count on the contrarians to shed some light that I might otherwise be ignorant to.
I think everything would be okay if everyone just reminded themselves to be civil and not to use downvotes to disagree.