Long Rant on Playability

Here's a problem I've been having with RPGs in general, one that has started to bug me about modern ones in particular. I like modern rpgs. In general, there is less problem with descriptions, background and the like than in other genres (though if you want to get a city right, it can be far more research intensive). The problem is the whole issue of playability. In fantasy settings this is rarely as large an issue - everything is archetypal, and most people know the basic conventions of the genre.
In modern games, this isn't as much the case. You want to play a police office, for example. How much do you really know about the law of the campaign area? Or how laws get enforced in that country? Is having watched Cops and Law & Order a lot enough to make up for this? What about a 3000 year old Vampire who, since she's now a PC, is going to do stupid things like risk death? Or the high-paid corporate assassin?
To a certain extent we can cover this with some fiction novels but what about more mundane things, like a economics professor at the local university? At some point, what is allowable in a system collides with what is actually playable by the player. And in that sense the university geology professor is as much a strain on playability as the werewolf living out his life as a animal tamer for the local zoo.
The other problem this raises is credibility of the setting. Characters who do things that it seems blatantly obvious that they wouldn't (like a military covert ops agent deciding to put the corpses of his friends in the back of a car while driving away from a cemetery*) strain the believability in their role and make the setting a bit less real. Every bit helps.
So should people only play what they know? Of course not. That would be boring, for one thing. But should they play something they have no idea about? I've done it, and I'd say no.
Any other thoughts?

* Though games that deal in weird horror (like UA) can allow for this since characters may well cling to normal rituals at the sight of Things Men Was Not Meant To See(tm) or be under the control of Something Other(patent pending), but even so one can only go so far before you hit absurdity and screwball comedy...