Some changes to the software are more requested than others; here's a review of the status of five feature requests and bugs affecting the English Wikipedia that I consider to be important, here given in chronological order by the time they were requested or reported.

Documentation (bug 1)

The first request that was reported on the current bug-tracking system was for better documentation. New features are added to the code all the time; developers tend to be busy, and so the documentation often lags behind. You can help with the documentation; Help information for using most of MediaWiki's features is stored on MediaWiki.org, and as with all wikis it will benefit from constant improvement.

Single user login (bug 57)

The most requested feature request ever, single user login would allow for usernames to be valid across all Wikimedia projects and languages, and not just valid on one project in one language. There was a test run in November 2006 which seems to imply that the project is feasible; the most recent update on its status was this message from June by User:Brion VIBBER, which lists the remaining work to be done as "Further work, testing, test rollout, separation of private from public wikis, live rollout." At the moment, separate accounts have to be registered on different projects; you can help simplify any future transition by using the same username and email address if you create an account on a different Wikimedia project or a different language as you do on the English Wikipedia.

Section edit link for section 0 (bug 156)

Every section on a Wikipedia article has an [edit] link to allow editing of that section, except for the introduction ('section 0', the section before the first section heading on a page). The main problem with implementing such a link is disagreement about where it should be positioned; there have been suggestions to place such a link as a tab (like 'edit this page' is), on the same line as the title, and on the line just below the title. This feature apparently existed in some skins in the past, but was removed, due to interference with right-floating elements in the page's content. Several workarounds exist; there are five user scripts and two templates (see Help:Section#Editing before the first section) designed to fix this problem, and it's possible to manually enter a section of 0 to edit by editing a different section and changing the section number in the edit page's URL to 0.

Subcategory paging in large categories (bug 1211)

At the moment, in a large category that contains both categories and non-category pages, sometimes the subcategories will end up in the second page or subsequent pages of results, which can be counterintuitive and inconvenient. This is the most requested bugfix ever; at least two alternative versions of the code that solve the problem have been written, but neither has yet been applied to the main code by developers, and as both versions are over a year old it's likely that some changes would be needed before they could be used with the current codebase. At present, one workaround (such as is used on CAT:AFD) is to use category sortkeys to sort subcategories to the start of the list, thus making sure that they are visible on the first page of results.

New-messages bar problems for anonymous users (bug 9213)

When a message is sent to an anonymous user's Talk page, sometimes the orange 'you have new messages' bar will not come up; sometimes when it does come up, the bar will not then disappear for the anonymous user (not even after a refresh, cache bypass or after deleting cookies), staying up permanently and annoying the user. The bug therefore means that it's impossible to send a message to an anonymous user in such a way that they will be guaranteed to see it. Further information on Wikipedia is collected at Wikipedia:Administrator intervention against vandalism/Bug ID 9213; the bug has not yet been fixed because it's not certain what is causing it (although it is known to be a relatively recent bug), and as an intermittent bug it's hard to pin down.