Sitecore made a pretty big change to the default configs if you are using xDB on your site. By default now, Sitecore will only track users that have been identified in xDB. By "identified," we mean that the user has logged in, clicked a link from ExM, or in some form had the Identify(identifier) applied to their session.
If you are like me, and you are tracking people based off some of their anonymous data, like IP, company, location, etc... having this off is really important. So I must make sure I patch this from now on for all of the instances that need anonymous tracking.
Sitecore's rationale is improved performance and overall de-cluttering of the index. Since every single session is in xDB, it gets really, really busy.
To improve performance and optimize indexing, in the Sitecore.ContentSearch.Analytics.config file, we changed the default value of ContentSearch.Analytics.IndexAnonymousContacts setting from true to false. We also updated the description of this setting to explicitly state that false is the new default value.
But if you are not a site that has an e-commerce presence or a subscriber userbase, then knowing who is on the other side is difficult. You have to rely on goals, events, and anonymous user tracking to do personalization.
The config file change for this is Sitecore.ContentSearch.Analytics.config. The setting is
<setting name="ContentSearch.Analytics.IndexAnonymousContacts" value="false"/>
Setting this to "true" will start tracking all users again, including anonymous.