From a post I made a while ago in another thread, It's the structural strength of the A-pillar, extremely important in a roll over. Check out how the B-pillarless E-class beats out its competitors and even the 5 year old C-class too!
Model------------------Curb weight (lb)------------Peak force (lb)----------Strength-to-weight ratio
Mercedes E class 2-door
2010-11 models-----------------3,736---------------------20,852----------------------5.58
Mercedes E class 4-door
2010-11 models-----------------3,880---------------------20,961---------------------5.40
Volvo S80
2007-11 models-----------------3,756---------------------15,989---------------------4.26
BMW 5 series
2011 models---------------------3,803---------------------15,364---------------------4.04
Mercedes GLK
2010-11 models-----------------4,157---------------------26,647---------------------6.41
Volvo XC60
2010-11 models-----------------4,172---------------------21,828---------------------5.23
Audi Q5
2009-11 models-----------------4,237---------------------18,684---------------------4.41
Mercedes C class
2008-11 models-----------------3,510---------------------18,826---------------------5.36
Audi A4
2009-11 models-----------------3,523---------------------16,197---------------------4.60
Source: IIHS