Crowd-wise, DLR is pretty crazy, and the crowds are packed into a smaller area than at WDW.
Otherwise, in terms of overall atmosphere.....One word: magical!! Actually, to be honest, it is far more than just one word! It's also enchanting, stunning, beautiful, festive, extremely detailed and immersive, a feast for all the senses, etc. It will practically bring tears to your eyes!
I could go on explaining in words, but I think you will find our "Disney at Christmas" thread (link below) tells the tale the best and most effectively. This is a thread that was started as a simple query of what DLR was like during the holidays, and it became sort of a one-stop shop for all things DLR-Christmas, with all kinds of information on what exactly you can see/do/experience at DLR during holiday time as well as photos galore of the sights, the food, the holiday ride overlays, the Christmas trees, the merchandise, etc. You can really feel the love that we DIS-ers have for DLR at Christmastime in this thread. So thank you for giving me a chance to introduce someone new to it!! Here is the link:
http://www.disboards.com/showthread.php?t=2262171