I also have a Bernina 630 that I totally love. It was the best choice I coujld have made for myself.
My Must Have Features & Why:
Needle up/down: Use this all the time.
Lowering the feed dogs: For free motion quilting
Adjustable needle position: Often makes a sewing task easier
Speed control: There are times when I want to sew very slowly.
Mirror Image: So I can flip stitch designs to sew in the opposite direction.
MY FAVORITE FEATURE:
Infinite stitch length and width: For me, the predetermined width setting increments of most other computerized machines are too large, the gaps too big. I like to do invisible machine applique, the total control of infinite stitch and width really makes this possible for me.
Features that could be a must:
IF I was an avid paper piecer, the thread cutters might be a must have.
Good luck with your sewing machine hunt! :)