When I watch it in reruns, it always amazes me how many bit players, in one shot parts, later went on to be quite successful in their own right.
For example
Helen Hunt - Murray Daughters
John Ritter - Priest that married Ted and Georgette
Henry Winkler - Rhoda's date
Fred Grandy - Reverend at the Teddy Awards
Bernie Kopell - Mary's date
Craig T Nelson - Mechanic
Louise Lasser - Bank clerk (totally stole the scene)
Beth Howland - Separated wife of man Mary Dates
Isabel Sanford - Mother to a coworker Mary interviews
David Ogden Stiers - Station manager at WJM
Gordon Jump
Dabney Coleman - Congressman
Mary Frann - Mary's anti-Semitic friend
Joseph Campanella - Mary's date
Mary Kay Place - Mary's neighbor at her new flat (along with established Penny Marshall)
Gordon Jump - Tried out for sportscaster
Mackenzie Phillips - Mary's "little sister" in the "little sisters" program
Jeff Conway - An ice skater used to make Lou jealous
Richard Kline - Attorney attempts to prosecute Mary for not revealing sources
Any others? I mean those who later became stars but had not had significant stardom prior, like Penny Marshall, Beverly Garland, Sheree North, Bill Daley and others like that.