much smaller
lenses are much smaller and generally better value for money with better optical quality
hand held night photography (12mm f/2.0 lens with the IS is brilliant!)
hand held long exposure water movement scenes when you forgor your tripod (any wide angle lens with the IS will give down to about 1/2 sec exposures)
hand held infrared photography at low ISO
tripod mounted long exposure photography thanks to unique Live BULB feature
much easier to use with manual focus lenses
availability of lovely sharp, wide aperture, compact lenses such as the
Olympus m.ZD 12mm f/2.0 lens, indeed you CANNOT buy an autofocus wide angle prime lens at all for a Canon or Nikon cropped sensor dSLR under $1500 which will cover wide angle wider than 30mm focal length (in 35mm terms), let alone have it wide aperture and image stabilised!!!
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can use old rangefinder lenses (eg. Leica) as well as Canon EF lenses albeit with limitations such as manual focus only (but new Birger adapter will probably allow AF and aperture change on Canon EF lenses).
availability of the f/0.95 and f/1.1 Cosina Voigtlander Nokton lenses
when using the Canon EF 85mm f/1.2L lens or 135mm f/2.0L lens, accurate manual focus is easier and it becomes image stabilised, as well as giving 170mm / 270mm FOV respectively - what an awesome low light indoor concert combination if size is allowed, otherwise use the Olympus OM 135mm f/2.8 (just can't AF or change aperture on the EF lenses until the Birger adapter is released)
accurate manual focus of tilt-shift lenses such as the Canon 90mm TSE is much easier
ability to convert almost any Nikon lens (or some other lenses) to a tilt or shift lens via an adapter
built-in image stabilisation 5 axis, 5EV works with ALL lenses and even in movie mode as well as during magnified manual focus
MUCH easier Live View mode, makes manual focus and video much easier
just touch subject on screen to immediately AF and take photo
eye detect AF
faster AF for slow moving subjects
35 point AF not just 19 points, and far easier to select an AF point using touch screen than navigation buttons
9fps burst rate instead of 8fps
5 frame exposure bracketing option not just 3 frames
ISO to 25,600 instead of 12,800
tilt OLED touch screen (7D has swivel LCD but no touch)
ART filters
multiple exposure mode
Timed BULB and Live BULB modes - timed shutter to 8 min not just 30sec
high quality 20mbps .MOV HD video mode
optional Bluetooth file transfer to phones
no need for microcalibration of AF with each lens as AF is much more accurate
nicer jpeg colours
Olympus flash system is far simpler to use than Canon or Nikon
MPO 3D stereo mode
no mirror, so quieter and no need for mirror lock up to reduce camera shake in high magnification situations