Right now, we can lockdown a few things here and there. But we don't have anywhere near something as simple and effective as OS X's parental controls.
In fact, if I was a novice with Ubuntu I would have no idea to look under users and groups to lock down an account. And, who is actually going to read all the information crammed into the sidebar of Authorizations?
There is an application called Pessulus, also known as the lockdown editor, that is a step in the right direction. But right now, there is no solidly easy way to administer a system in the same incredibly easy way as OS X.
Firstly, I propose we include an application like Pessulus by defualt. Secondly, I propose that we strive to enable it to manage the following:
+Blacklists and Whitelists for the Web(Firefox), E-mail(Evolution), and Chat(Pidgin).
+Disabling any individual applications completely.
+Easy Time limits such as: 2 hours, anytime from 10am to 10pm, Monday-Friday, etc.
Another possible function is to have controls in place during certain times of day. In the home, this can correlate to the presence of the parent. And, in a corporate setting, this can correlate to employee lunches.
This not only would benefit those with innocent eyes in our homes, but it could also be applied in a commercial setting!
