Like C.S. Lewis and Andrew Greeley L'Engle helped shaped my concept of what it means to be Christian. For the most part her characters' faith existed in much the same way mine does, as a solid background and a daily striving, not as an insecure need to prove something to others.
I appreciated that she didn't talk down to her YA audience when I was of the target age and it makes re-reading her work as an adult just as pleasurable as it was then. I learned about Mitochondria from her long before it was covered in any class and knowing what a tesseract is is still a recognition sign between nerds.
And as a nerd I really, really appreciated and loved the character of Charles Wallace. I wish she had written more with him.
In a time when the biggest kids series is the Harry Potter books, a series that L'Engle complained lacked any depth, it's even sadder to lose a writer who wasn't afraid to tackle the big issues that kids have to come to terms with growing up: faith, evil, death, standing up for a principle even if it means going against society's norm, and love. It always came back to love in all it's forms with her.
Wiki has a halfway decent introduction to her and her work.