Can users/authors be informed about content changes by email?
Emails are sent out when certain pages are edited/changed. This would typically be something users can configure on an individual preference basis, perhaps with control over which wiki pages are 'watched'.
An alternative is to watch the RSS feed of the wiki using a tool such as rss2email, and send emails with changes to a mailing list. This doesn't involve individual preference, however. Theoretically it is possible to have a client-side setup involving per-page RSS feeds all monitored by separate rss2email subscriptions. This is too complicated for most people, however. If receiving emails about changes, is something you require then generally you should look for a wiki software which supports the subscription of individual pages and the management of these subscriptions.
Some different but related email features…
If you can register a username/password on a wiki system, then in theory there should be some procedure for when you forget your password. This is a standard web application feature, which usually takes the form of a 'forgot password?' link, leading to the option to have your password emailed to you (or in the case of better one-way encrypted password systems, a new generated password is emailed)
Actually forgotten password procedures are only important if a community grows to spectacular size, such that emailing the administrator to ask for help, is no longer a sensible way of dealing with forgotten passwords. As such, the feature is perhaps a good indication that the software can (and does) support very large communities e.g. MediaWiki
Another standard web-app feature which appears in some wiki user-management subsystems. Requiring users to have a valid email address, is done by forcing the user to go check their email to get a code (in a link) to complete the registration. This makes the registration process more awkward, but can be a good way of reducing anonymity (kind of). Users can still create 'temporary' email addresses to get around this, but generally speaking it raises the bar for acts of vandalism/spam, and means you have a working email address to reach users.
Of course the normal wiki philosophy is to be a lot more open than this, and to allow editing by completely anonymous users who are perhaps not even logged in.