I guess no matter how old someone is when they die if they had a big impact on you you wish they had had more time to do whatever it is that they did. I still haven't read all of L'Engle's books, some of them are out of print, some the library just didn't have when I looked, and I sort of got side tracked from my quest to read them all a few years ago and somehow never started up again so I still have fresh works to explore but it's sad to know that they are now a finite set.
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.
http://en.wikipedia.org/wiki/Madeleine_L'Engle