I would like to echo this, if only to provide some balance to all the complaints. Not to deny that unwarranted (or at least poorly explained) reversions and deletions occur, but for a lot of these claims, upon closer examination, it turns out the edit had problems and violated one of Wikipedia's policies (most often because it's not the kind of information Wikipedia includes, with lack of sources the second-most-common reason). For some of the Slashdotters complaining, I would be astonished if their edits did in fact adhere to Wikipedia policy and style, just given the way they post here.
I've written at least a half-dozen Wikipedia articles or major article sections on a few different topics (bios of various authors, among others). None were summarily deleted or reverted; as far as I can tell (I don't keep close watch on them), they all still exist, and most still incorporate significant amounts of my contributions. They all seem to be in better shape now than when I first wrote them.
Here's what I did: First, I registered an account. Whenever I was thinking of making major changes/additions to an existing article (in some cases it was non-existent or a stub), I first posted on the talk page, expressing my interest in the topic, diplomatically describing what I thought could be improved, outlining what I was planning to do, and asking for comments and suggestions. This rarely got any replies, but ensured I wouldn't be stepping too hard on any toes.
In parallel with this, I figured out what the best available sources on the topic were: online material, sure (more for orientation than for incorporating), but preferably published books. If I didn't have them already I got hold of them, either through the library or second-hand (or Google Books if not in copyright). In some cases, articles in academic journals were more useful, and I was able to access these through a university library. I read or at least skimmed them.
Then I wrote the contribution, using (not copying verbatim) the information from the references and providing citations to them. I tried to write in a straightforward but encyclopedic style, to format it to be consistent with the rest of the article, and to follow the letter and the spirit of Wikipedia rules. I submitted my edits, and sometimes made another comment on the talk page, inviting comments or explaining any deletions I'd made.
I'm not claiming this is a guaranteed way to get your text on Wikipedia, but in my experience it worked. While I've also had some negative Wikipedia experiences (with smaller edits to other pages, where I didn't go through such an elaborate procedure), overall I find that as long as you understand the basic Wikipedia principles (what kind of stuff belongs and doesn't belong), can separate yourself from the work enough that your edits aren't blatantly about your own ego or biases, have solid sources for what you write, and can string a coherent sentence together, people will usually take you seriously.
There are many disgruntled attempted-Wikipedia-editors out there, some with more legitimate grievances than others. Sometimes it seems like most people who have tried to contribute got shot down. But of course, the most embittered ones are the ones most likely to go on about it: the real distribution could be quite different. Or maybe it's simply that most people aren't capable of making good, suitable contributions to an open encyclopedia. Would that be particularly surprising?