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Links in this essay will take you to information about the usage experts and their work. Numbers in parentheses are page references.

To read about this topic in A Writer's Reference, see section P1-e.
that versus which

H. W. Fowler suggested that “if writers would agree to regard that as the defining [restrictive] relative pronoun and which as the nondefining [nonrestrictive], there would be much to gain both in lucidity and in ease” (774). Years later, Fowler’s suggestion has become a rule for many writers. That rule is hotly debated, however, with some experts arguing that it is at best a rule of thumb that allows exceptions.
Why do many writers find the distinction useful? Consider the following sentences, given by Bryan Garner:

All the cars, which were purchased before 1995, need to have their airbags replaced.

All the cars that were purchased before 1995 need to have their airbags replaced. (648)

Both sentences are correct, but their meanings differ. The first sentence means that all of the cars were purchased before 1995; the second means that only some of them were. The difference in meaning could amount to real dollars for the company paying for the replacement.
How do we readers perceive the difference in meaning? First, the presence or absence of commas signals the writer’s intended meaning (unless the writer has punctuated the sentence incorrectly). Second, the writer’s use of which or that may reinforce the meaning, but only if the writer follows Fowler’s rule.
Let’s see what happens when a writer chooses not to follow the rule. Here is an actual example—one that caused much debate between moderate and conservative Republicans at their convention in 1984:

We therefore oppose any attempt to increase taxes which would harm the recovery.

The absence of a comma before the which clause sends one message to readers: We oppose only those tax increases harmful to the recovery. The choice of which could, for some readers, send the opposite message: All tax increases are harmful to the recovery. Adding a comma before which would express that meaning unambiguously. Those Republicans who opposed all tax increases won the battle—simply by inserting a comma before which.
Clearly the distinction between which and that is useful. But do we have to observe it religiously? Some experts don’t think so. They agree that we must use which when our meaning requires commas, but when our meaning requires no commas, they find either that or which acceptable.
R. H. Copperud goes so far as to say, “The prejudice against which with restrictive clauses [those with no commas] is just that, however—a prejudice” (347). He feels that placing so much emphasis on the choice of words distracts writers from the real issue—whether or not to use commas. The New York Public Library Writer’s Guide to Style and Usage makes the same point. It cautions professional copyeditors against conducting “which hunts” (blue-penciling every which without a comma). Far more important is determining whether the punctuation supports the author’s intended meaning.
One argument in favor of allowing an occasional which for restrictive clauses is based on style. When a sentence already contains that, the word’s recurrence can sound awkward. Wilson Follett provides a wonderful, although farfetched, example:

We believe that that machine that we built that year does just that. (324)

Adding a which helps relieve the monotony: We believe that that machine which we built that year does just that.
Even in less extreme cases, using more than one that can at times be awkward. The American Heritage Book of English Usage gives this example: “We want to assign only that book that will be most helpful” (39). In this sentence, the editors of the book suggest, which simply sounds better: We want to assign only that book which will be most helpful.
Conclusion: When you are writing for a publication that recommends observing Fowler’s strict distinction between which and that, by all means comply. Otherwise, observe the distinction unless doing so results in an awkward sentence. Above all, make sure your punctuation expresses the meaning you intend.