Common article practices I dislike

Unhidden navigation templates

I hate these. They take up a huge amount of room and display tons of links in the hope that they will interest the reader. They are just like spam.

The way to use navigation templates is to use show/hide. See User:Ideogram/navigation templates. A user can click if they are interested without it taking up too much room. If you have multiple navigation templates, they should all be hidden behind one button, as in the example.

Fixed image sizes

Wikipedia has a feature under "my preferences" that lets users select their preferred thumbnail image size. Specifying an explicit image size in the text breaks this. Not to mention sprinkling randomly sized images around almost always looks bad. Just remove the size specification.

One exception to this rule would be when an image needs to be set larger to show detail.

Linking terms multiple times

I believe this is against official Wikipedia style.

Dates

Full dates should always be written February 23, 2007. Individual dates like February 23, lone years like 2007, and decades and centuries like 1990s and 1900s should never be linked.

Redirects

Ideally, no article link should ever be a redirect. If you create redirected links by moving an article, fix them. Often an article will link to a redirect that actually is irrelevant to the original article. These should be unlinked, not just fixed.

Too many links

Don't link to some broad concept that just happens to be related to the word you are using. Example: linking "capital of China" to a list of all the capitals China has had. There are no rules for this, try to use common sense, if you have any.

Too many images

Wikipedia is for people who can read. One image per substantial subsection is sufficient. If you have a lot of short subsections and the images from them start piling up you have a problem.

Excessive use of bold/italic

As far as I can tell, only one term should be bolded, the name of the article. Use of bold/italic can almost always be replaced with quote marks.

People don't know about <blockquote></blockquote>

I see lots of people trying to format large quotes in awkward ways.

Excessive and overgeneral categorization

Don't put an article in every category you can think of. The category system is organized into subcategories, try to put the article in the most specific categories you can find, since those will be included in the more general categories. Putting an article both in a category and a subcategory of that category is almost always wrong.

Unsorted see also links, external links, and categories

If you can't think enough to sort the links when you add one, you probably don't know what you are doing.