My Wikipedia contributions are basically minor grammar fixes. I also sometimes fix facts, but get reverted by small children.
I am not a fan of Wikipedia as an institution, with its stifling bureaucracy and myriad conflicting guidelines, but I do think it is a really useful and interesting resource, and I spend a lot of time reading articles about anything. (HIGHLY RECOMMENDED: Wikipedia:Unusual articles.)
Run-on sentences with "however": e.g. "He lost his fortune, however he was never truly poor" should be "He lost his fortune; however, he was never truly poor."
Term-referent confusion: e.g. "A horse is a term for a quadruped" should be "A horse is a quadruped", or "Horse is a term for a quadruped".
Failing to close a subclause with a matching comma, e.g. "John, also known as Bob is a man" should be "John, also known as Bob, is a man".
Overuse of commas in adjective lists, e.g. "little, green men" should be "little green men". (There's no pause in speech.) This is particularly the case where we have a noun adjunct: "used tractor parts", not "used, tractor parts"; or "a 2D platform game", not "a 2D, platform game".
Misuse of commas when naming a specific thing, e.g. "the film, Jurassic Park" should be "the film Jurassic Park". (This makes sense if you read Relative_clause#Restrictive_and_non-restrictive.)
Misuse of commas regarding restrictive clauses: "the fattest person who lives in England" is not the same as "the fattest person, who [by the way] lives in England".
Spurious commas before quoted material that isn't reported speech, e.g. he calls himself, "Batman".
Use of inappropriate verbs with direct speech, e.g. he criticised that "it's a bad play". This is particularly rife in video-game reception/review sections where people are desperate to make "X liked it, Y liked it, Z disliked it" sound interesting.
Inclusion of full stops (periods) in quoted material in mid-sentence, e.g. I said "Go away." and...
Lists of things that don't all work in the grammatical context, e.g. "I have been a firefighter, a gardener, and worked in finance" (which expands to "I have been worked in finance"). I have seen good writers do this in print, so perhaps it's acceptable, but it's at least unclear.
Failure to use the past participle when needed, e.g. "In 1990 he announced he released a new song" should be "In 1990 he announced he had released a new song"; and, conversely, using it when not needed, e.g. "On 11 November 2004 the site had closed down." I rather suspect that this comes from the tense being modified by successive editors as time passes.
Weird extraneous hyphens with Adv+Adj, e.g. a potentially-disruptive event (do these people write a very-nice day?).
...and omitted hyphens, e.g. "two year-old children" and "two-year-old children" are quite different things, though you'll get arrested for fucking either.
Ever made a perfectly good edit to Marillion while not logged in, and some 13-year-old moron whose daddy probably wasn't even alive yet reverts it, without knowing anything about the topic, because "hee hee I've got the tools"? Right! That's Twinkle, or Huggle. This satirical article explains it quite well (until someone changes it due to OH NO OFFENSIVENESS): Wikipedia:Edits Per Day
Be very, very familiar with all the non-policy hints like WP:BEANS
Make friends with everyone else who is familiar with these things
Learn them inside-out so that you know the ones for both sides. It's like proverbs. For example: "a stitch in time saves nine" (you should have done it earlier) is the opposite of "don't count your chickens until they're hatched" (you should wait until later)
When there is an argument, choose the appropriate non-policy
If your opponent is wise enough to choose the opposing non-policy, remind them that these are not policies
There's a sort of entropy factor involved. Suppose that I fix some common error that most people incorrectly disbelieve (imagine some kind of Snopes thing, or a piece of grammar with "whom" or commas or whatever, or transgenderism HA HA JUST KIDDING?). And naturally people will think, wrongly, "this is an error" and they will revert or edit it in some way back to how it was. This is the entropy goal. But it may be actually incorrect. Now it's hard to say how we should deal with this. You might insert some sort of HTML comment like "DO NOT CHANGE THIS", which is obviously stupid for obvious reasons; or you might, um, anyway. I do feel things are going to gradually gravitate to what people expect rather than what people deserve (or what is true, which I like to imagine is the same as the latter, but perhaps I'm an optimist).
LATER COMMENT: what the hell is this? I must have written this drunk. But it has the word "entropy" in it so it's great.
MUCH LATER COMMENT: I think (a small part of) what I was trying to say is that you can spend six months trying to get an article into a certain state, with proof and arguments and votes etc., but if most people think otherwise (even if they are demonstrably wrong), they will swiftly push it back again. Homeostasis!
