What are the big features that the 5 series has that the Model S won't?
Tesla hasn't announced the options list yet, so this is just my guess:
adaptive cruise control
side view cameras
all around camera (top down) cameras
head up display
self closing doors & trunk
power headrests
AWD (although Tesla might add it in future model years)
4 zone climate control
4 wheel steering
active sway bars & shocks
night vision
There's more, these are just off the top of my head. I've never owned a BMW and don't know much about them. Maybe a 2011 BMW 550i owner can chime in and add to this.