Thread cutter, Up Down, Auto foot lift or knee lift, wide throat space, walking foot..build in...or attached. The smaller "built in" they are really not build in, they attach but much much smaller than the chunky white foot that you can't see around, Auto tension (most machines have that now). These are the key features that I like. You may like other features like lots of decorative stitches. I have enjoyed the Janome machines with the 9mm rather than 7mm.
But for quilting, you don't use this feature much.