Manual Focus Mode You can use the manual focus mode of your camera to Zone Focus which is a popular technique used in street photography.
Live-view Live view lets you take photos without using the viewfinder which makes you less obstrusive and this works very well during street photography where you don't want to get people's attention.
28 mm - Good Wide Angle Coverage Having a good wide angle coverage becomes very important in street photography.
RAW shooting capability RAW files contains much more color information compared to JPEGs, and gives you more room for post processing without significant quality loss. They provide wider dynamic range and option to recover shadow and highlight clipping to some degree.
Small Body Small body of Sigma DP1x will get less attention from the people in streets and give you an amateur look, which is indeed an advantage for street photography.
Large APS-C (20.7 x 13.8 mm) sensor Large sensor in Sigma DP1x 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.
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, especially with heavy camera bodies and lenses. Besides, LCD screens are very hard to read under bright sunlight.
No Image Stabilization An articulated screen gives you the flexibility to shoot in non traditional positions such as waist level, which helps you to be more creative and unobtrusive in the streets.
Slow Lens at Wide: f4.00 Lack of Image stabilization may result in blurry photos especially in long exposures and at long focal lengths