Autobiographical articles. One of the most common mistakes for newcomers is creating an encyclopedia article about themselves. Because Wikipedia is an encyclopedia, it is not expected to have a biographical article about every person who contributes. Your user page, however, is a perfect place to write about yourself; just click your user name at the top of the screen when you have logged in and edit to your heart's content.
Company articles. It is often better not to write an article about the company you work for or own. Firstly, you may have problems maintaining a neutral point of view, and secondly, it may be that your article will be quickly deleted. If your company is notable enough, someone else will write an article about it. (See Wikipedia:Business' FAQ.)
Dictionary-type entries. We take the stance that Wikipedia is not a dictionary. Each article should aim to cover its topic beyond a simple definition and teach something about greater context. Pure dictionary definitions belong in our sister project, Wiktionary.
Redundant articles. Before creating a new article, run a search for the topic — you may find a related one that already exists. Consider adding to existing articles before creating an entirely new one. In searching keywords, remember that article titles are usually singular, e.g. "Tree", not "Trees". Also attempt a search on Google of the form "site:en.wikipedia.org <topic>"; articles may be missed by a Wikipedia search but caught by Google, especially if the terms you choose are not present in the article title. Google's spelling suggestion feature also helps a great deal. Redundant articles often result when a user comes across a red link, so see Wikipedia:WikiProject Red Link Recovery for more details. A good principle to remember is that Wikipedia is very inclusive and has an entry for almost everything, so chances are almost any subject already has an article. Also, an article for a red link may have been deleted, so see Special:Log/delete and search for deletions to find out when and why an article was deleted.
Articles which are too short to have encyclopedic value. Articles must establish the context and notability of the subject. If an article does not contain enough content to classify it as a stub, then it may qualify for speedy deletion. Instead of creating a very short article, consider adding more content to the page before saving it, or using the {{inuse}} tag to indicate that the article is in the process of expansion.
Deleting...
Deleting useful content. A piece of content may be written poorly, yet still have a purpose. Consider what a sentence or paragraph tries to say. Clarify it instead of throwing it away. If the material seems mis-categorized or out of place, consider moving the wayward material to another page, or creating a new page for it. If all else fails, and you can't resist removing a good chunk of content, it's usually best to move it to the article's "Talk page", which can be accessed using the "discussion" button at the top of each page. The author of the text once thought it valuable, so it is polite to preserve it for later discussion.
Deleting biased content. Biased content can be useful content (see above). Remove the bias and keep the content.
Deleting without explanation. Deleting anything that isn't trivial requires some justification, or else other users who care about the article's development will be caught unaware, and may think you're being intentionally sneaky. It is best to put a few words in the edit summary, or else you can simply write "See talk:" in the edit summary box and explain on the talk page.
Deleting or removing text from any Talk page without archiving it, except in your user space. Talk pages or any discussion pages are part of the historical record in Wikipedia. Every time the pages are cleaned up, don't forget to store the removed text in its corresponding archive ([[/Archive]]) page. (See Wikipedia:How to archive a talk page.)
Contributing...
Poorly structured lead sections. The lead should establish context, summarize the most important points, explain why the subject is interesting or notable, and briefly describe its notable controversies, if there are any. It should not "tease" the reader by hinting at but not explaining important facts that will appear later in the article. (See Wikipedia:Lead section.)
External links in text. Relevant external links should be added to a links section at the foot of the article. If the link is a reference to a reliable source, then you should use reference tags to create an inline citation. (See Wikipedia:External links.)
Signatures in articles. The need to associate edits with users is taken care of by an article's edit history. Therefore, you should use your signature only when contributing to talk pages, the Village Pump, or other such discussion pages. (See Wikipedia:Signatures.)
Over-doing it
Over-respecting "the author".
Criticizing instead of editing. Articles have no single author with one overarching plan. Offering a suggestion or criticism on the Talk page can be helpful, but if a page is not protected, it is often faster to just give the article what you think it needs.
Failing to be bold. Yes, you might mess things up a little. But someone else will probably clean up after you. Really, go ahead and change it. We mean it.
Over-capitalizing titles. For instance, the second word in "French grammar" is in lowercase. That's the Wikipedia house style.
Arming for war. Wikipedia is a unique community of reasonable and consensus-oriented people. In other words, this isn't Usenet, and flaming is severely looked down upon. For more about Wikipedia manners, see Wikiquette.
Getting annoyed because you find some bad articles. Wikipedia is, and always will be, a work in progress; please tolerate our imperfection, and help us improve. There are a lot of smart people here, and everyone finds they have something to contribute. If you're still skeptical, see the replies to common objections.
