"That" defines something, whereas "which" adds new information in a separate clause, often needing commas.

BBC: 10 questions on grammar

There are a few too many commas for Forbes, but learning to remove those takes years of practice.

FORBES: Forget What WikiLeaks Says About Heads Of State And Find Out How The State Department's Doing

He drives me crazy with his demands that I add commas and semicolons.

FORBES: My Secrets: How I Became a Prolific Writer and Learned to Get Beyond School Essays

Markers also found candidates used commas and capital letters randomly and were uncertain about the correct use of tenses, apostrophes and paragraphs.

BBC: Scottish exam markers find errors in basic English

Mayor JOHN LOSCHKE (Parachute, Colorado): Parachute, Colorado, at that time had a population of somewhere in the neighborhood of 300, 3-0-0, no commas, no nothing.

NPR: Industry Seeks to Extract Oil from Rocky Mountain Shale

Some of his most entertaining passages pertain to his editing by the New Yorker, whose fussy style guidelines and passion for commas drove Thurber wild.

ECONOMIST: Letters

Making a game would be like writing this post with 4, 000 commas that all had to be in the right place to keep the screen from going blank.

FORBES: If You Think Most Games are Dumb, You're Playing Them Wrong

From the moment of painting the figure 1 until the day he died, when he had reached well past 5500000 (no commas marred his work), his daily task was painting numbers and whispering their names, eventually into a tape recorder.

ECONOMIST: Roman Opalka