Although MediaWiki is a feature rich wiki software there are still some important common features missing. Luckily you can add them via extensions to MediaWiki. So here a list of plugins that are very useful in any MediaWiki installation.
In a usual MediaWiki installation you have no possibility to determine, when a particular user has registered an account. There is just the „Special:Userlist“ special page that lists all currently existing users alphabetically (with additional user rights such as bot, sysop and bureaucrat flag). So things like newbie welcome messages and quick reaction to certain vandal accounts are overly complicated if not even impossible. Luckily the Newuserlog extension solves this very nicely by adding a new user log feed into the central MediaWiki logs at „Special:Log“. You can even filter the log and access the new user log directly by accessing „Special:Log/newusers“. IMHO the new user log should be directly builtin into MediaWiki and not an extension.
As well it is not possible to rename a user in plain MediaWiki. Sooner or later in every wiki there will be users that bug you to rename them to a new user name because they want to keep their old contributions associated to them, their existing configuration, their probably very long „watchlist“ and other things. Or you will get some vandals with very inapropriate user names, you don’t want to see anylonger anywhere in your wiki. With the Renameuser extension you can add this often requested feature to your wiki. However don’t forget some small pitfalls: By default only people with „bureaucrat rights“ are able to rename a user and after renaming the user you still have to move the old user page(s) to the new name manually.
Spammers like wikis. They insert their ads full of web links into wikis using robots so that you have quite a lot to do in case such a bot hits your wiki. Although MediaWiki flags every weblink with a „nofollow“ tag (which effectively avoids exploiting a wiki as a link farm for pushing a web site in search engines) dumb spammers probably need some further years to recognize that spamming MediaWiki is useless for them. In worst case you maybe want to close your wiki for guest contributions just because of spamming, which is odd. So the ConfirmEdit extension asks for solving a simple captcha in case a guest (person that does not have a user account) tries to add a new weblink to a wiki page (you can configure the exact behaviour within a broad range). This effectively reduces wiki spamming alot and thus avoids closing down your wiki for guests and of course ConfirmEdit is quite unobstrusive and doesn’t bug people on every edit.
Browsing categories in MediaWiki isn’t very intuitive and one gets quickly lost in the category hierarchy because there is no tree view of categories. The CategoryTree extension solves this by adding a dynamic view of the wiki’s category structure as an expandable tree directly at category pages (and on a special page and embeddable via a custom tag in article pages), which makes browsing categories much faster and makes organizing and maintaining your category structure a lot easier.
When using templates with variables or in case you need some templates reacting more advanced to certain conditions you quickly will notice that templates are somewhat limited in MediaWiki. The ParserFunctions extension (see also the info page at mediawiki.org) provides programming like features such as „if“, „switch“, „for“, calculating („epxr“ function) and many more which are very useful when writing some flexible, advanced yet short templates for your wiki. Otherwise the desired functionality wouldn’t be possible or only using some very wired and complicated „wiki hacking“ with nested templates.