If I delete a document owned by me, everybody loses access to that document. However, if I delete a document owned by someone else, it is only deleted from my point of view. Everybody else still sees that document.
In an environment where everybody is responsible for maintaining a pool of documents, this is terribly confusing. How do I delete a document on behalf of another user so that the document is gone for everybody?
Edit For Google Apps accounts, this was resolved nicely in late 2018 with the introduction of the Team Drive feature. We've had good experience with the co-ownership use case I described above after moving documents into one.
Jury is still out on a good solution for personal accounts.