I'm normally a bit of a critic of arbitrary pronouncements on how to phrase or punctuate restrictive and non-restrictive clauses. Most of the time the meaning is perfectly clear from context whether you use that or which and whether or not you put a comma. Not always though. It can can make quite a difference.

Nick Clegg is being interviewed on TV at the moment. He was saying that we won't be able to judge the coalition until the end of it, five years from now.
He went on to make a list that took the form "We will have to see whether..., whether..., whether... .

For one of the items in the list he clearly meant to say

"whether we have an education system that helps every single child".

Instead, thanks to his word choice and the clearly audible pause that the comma would mark, what I heard was

"whether we have an education system, which helps every child."

That's quite a different thing and, given the scale of cuts they are likely to be making, probably far closer to the truth than he'd like us to believe.

It seems to me that clarity is the criterion for criticism, Bob. Where meaning is in jeopardy, a rule is appropriate.

Spot on, Geoff. Lynne Truss gave an excellent example in the title of her book, though generally I don't agree with her of course.