Wikipedian since October 2003.
Three major pet peeves
1) Not spelling out numbers from one to nine, where it is appropriate to do so (and it almost always is). I'd love to eliminate every reference to "4 lakes" and "6 composers" on this site. It should be "four lakes" and "six composers." Numerals in Wikipedia articles, for numbers under ten, look tacky; we should expect more from Wikipedia.
2) US postal abbreviations of states in articles that aren't about, well, postal abbreviations. "Lexington, KY" means nothing to someone outside the United States, and it looks dreadful. It's an address on an envelope as requested by the US Postal Service; it's not a real place. SPELL OUT STATE NAMES. Or omit the state names altogether on the second reference. Postal abbreviations don't belong in an encyclopedia.
3) Ampersands (&). This is an encyclopedia. There isn't any excuse for being too lazy to spell out the word and. Ampersands just don't cut it. The only exceptions are cases in which the ampersand is part of the original title, such as Laverne & Shirley, and even that one is arguable. If you're too lazy to spell out the word and, then I'm not really sure why you're contributing to an encyclopedia.
My current passion is REDIRECTS. Let's remember to make redirects! Redirects save time. They lessen the chances of two separate articles on the same topic. They help people find what they're looking for faster. And they're fun and easy! Here's to redirects!
This is how bad it is: unbelievably, there wasn't, until I fixed it, a redirect for the article on the famous artist Christo from the page "Christo" to the non-intuitive (for most users) actual article at Christo and Jeanne-Claude. Check "What Links Here" at that article to see just how many (dozens and dozens) of links were showing up red simply because no one, incredibly, thought to do a redirect from "Christo". 
In my opinion, you can't overdo it on redirects, not unless you're doing redirects where there should rightly be a disambiguation page. And, even in those cases, a temporary redirect page, until someone puts together a dab page, is better than nothing at all. Several times I've come across duplicate articles, each under a separate namespace but about the same person or thing, and it's just because no one thought to do an obvious redirect.
Let me say it one more way: Creating redirects is part of writing a new Wikipedia article. If you haven't done redirects, you haven't finished the job!
(Note: Since I wrote the above, I've been told there had been a Christo redirect, but that I just happened to come across the article at a time when it had been deleted, mistakenly, for about two weeks, as the namespace of the article moved. That makes me feel a bit better.)
More and more on Wikipedia I'm seeing this "notability" creature raise its ugly head, and I'm seeing the vague term defined more and more broadly. I fear that some users' obsession with notability -- or, more precisely, enforcing their standards of notability-- has the potential to turn Wikipedia into a place of two warring factions. I'm a firm believer in that now-tired but still-true cliche that "Wikipedia is not paper." What purpose does it serve to delete otherwise fact-based and readable articles about subjects that are supposedly "non-notable"? This is not, repeat not, a traditional encyclopedia, and we have the luxury to be able to include obscure and esoteric information that paper encyclopedias don't. I wouldn't actually call myself a rabid inclusionist -- I'm all for deleting vanity pages and the like -- but I've just seen too many examples lately, for my comfort, of people playing the "notability" card as a catch-all excuse to delete that which need not be deleted and that which does not harm Wikipedia.
Note: The "Arguments against deleting articles for non-notability" section of the article on notability I've linked above actually corresponds quite closely to my own beliefs on the subject.
A favorite quote that is relevant to Wikipedia
"The Internet has created the most precise mirror of people as a whole that we've yet had. It is not a summary prepared by a social scientist or an elite think tank. It is not the hagiography of an era, condensed by a romantic idealist or a sneering cynic. It is the real us, available for direct inspection for the first time. Our collective window shades are now open. We see the mundanity, the avarice, the ugliness, the perversity, the loneliness, the love, the inspiration, the serendipity, and the tenderness that manifest in humanity. Seen in proportion, we can breathe a sigh of relief. We are basically OK."