DBs, memcaches, tape backups, offsite storage, log files, etc.
Past that, deleting things from databases is sometimes hard. If, for example, I delete userX, and userX was the founder of a number of forums, or chat rooms, or groups, or facebook pages that are linked to userX? Do those groups and forums and things count as 'belonging' to userX? If userX happened to be the guy who created /r/news, do we delete that subreddit, and all of the content therein?
What if userX was a paying member? Do you delete all his old invoices? How do you make sure that doing so still allows you to balance your books?
There are indeed real world scenarios wherein just deleting a user and cascading that delete throughout the system breaks things. In some cases, it might be better to replace userX's personal details with 'AnonymousUserX', but then that might leave behind content they've generated, which you then have to replace with "DELETED CONTENT" or some other stub, which causes complications.
Past that, deleting things from databases is sometimes hard. If, for example, I delete userX, and userX was the founder of a number of forums, or chat rooms, or groups, or facebook pages that are linked to userX? Do those groups and forums and things count as 'belonging' to userX? If userX happened to be the guy who created /r/news, do we delete that subreddit, and all of the content therein?
What if userX was a paying member? Do you delete all his old invoices? How do you make sure that doing so still allows you to balance your books?
There are indeed real world scenarios wherein just deleting a user and cascading that delete throughout the system breaks things. In some cases, it might be better to replace userX's personal details with 'AnonymousUserX', but then that might leave behind content they've generated, which you then have to replace with "DELETED CONTENT" or some other stub, which causes complications.