Clarity grading consists of more than just visual identification of a diamond's inclusions and inclusion pattern. A diamond should be examined with the unaided eye and under 10x magnification so that an overall visual impression of the stone may be considered. They are normal characteristics and all diamonds have inclusions, even flawless ones when viewed under 20 - 30x magnification. Diamonds must be clean for accurate grading. It is quite easy to mistake dust, dirt and oil for inclusions.
Large inclusions tend to lower clarity grades more than small inclusions.
The type of inclusion can have a dramatic affect on the grade. Dark inclusions tend to lower clarity grades more than colorless or white inclusions. If it is easy for you to detect inclusions through the top of a diamond without magnification, it is almost certainly an I grade. Remember that your opinion of a diamond can be affected by the quality of diamonds you have available for comparison. If you compare an SI clarity diamond with the I-2 pictured above, the SI will always look considerably better.