First of all, I got all A rank, and all of the Dream Drops! YAY!
WARNING. EXTREME SPOILERS AHEAD.
Getting all the dreamdrops lets you play as Claris and Elliot! You simply jump into the fountain as Will or Helen, and you'll become the respective Dreamer. This is really cool, but they look kind of wierd with SUPER good graphics. Plus their eyes are HUGE. Hehe.
Second of all, getting all A-Ranks lets you listen to the entire NiGHTS 1 soundtrack in your soundtest. That's right. It's every bit as glamorous as it sounds, folks!

If you complete the game, 100%, then you get a snowy version of the Dream Gate. Snow's always falling, and instead of grass there's snow n' stuff. It plays Dreams Dreams: Sweet Snow (instrumental) while you're there. Fun!
That, my friends, is all. Also, I would to comment...
Dearest SEGA...
Thank you for the wonderful game. It's everything I hoped it would be, and more. We're glad you finally did something right.
Your Friend, Zeke
Oh yeah....I forgot...
JACKLE.