Large Four Thirds (17.3 x 13 mm) sensor
Large sensor in Olympus E-PL9 ensures a good low light image quality compared to smaller sensor, and it also provide shallow depth of field and a nice blurry background when needed.
Medium size Body
With relatively smaller body compared to DSLRs and better ergonomics compared to Compacts,Olympus E-PL9is a good compromise between size and functionality.
Carrying this weight all day may not be very comfortable for you.