Large APS-C (20.7 x 13.8 mm) sensor Large sensors provide shallower depth of field and a nice blurry background for your portrait shots.
RAW shooting capability RAW files contains more color information compared to JPEGs, so it is easier to get a natural skin tone and colours during post processing, which is very important for portraits.
External Flash Shoe Use of External flash creates many creative lighting opportunities for portrait photography. Techniques like bounce and fill-in flash will make your portraits stand-out from the crowd.
Manual Focus Mode You have to nail the focus perfectly for a good portrait shot. Compact cameras are not great at focusing at low light and complex lighting conditions, so manual focus mode is very useful at these times.
Manual Exposure Mode A correct exposure of portrait model is crucial. Even though modern digital cameras features advanced algorithms to calculate the right exposure, they still may fail some difficult conditions. Manual exposure mode in these conditions helps a lot in order to achieve correct exposure.
Very Low Resolution Sensor: 5.0MP Sigma DP1's 5.0MP sensor will not provide much room for cropping or large printing your portrait shots.
No Image Stabilization Lack of Image stabilization may result in blurry photos especially in long exposures and at long focal lengths
No Built-in Viewfinder In the lack of a a viewfinder, holding the camera away from the body while using live view is not very stable, especial with heavy camera bodies and lenses. Besides, LCD screens are very hard to read under bright sunlight.
Not so good Ergonomics&Handling Good handling and existence of well placed physical controls on the camera body is very important during portrait shooting. It is faster and more intuitive to change the camera settings with dedicated buttons and knobs instead delving into the menus on the LCD.