in digital circuits, there are only two levels - high and low. moreover the transistors work in linear region.
while in analog circuits there are no defined logic levels........signal can take any value.
Even the transistors work in saturation region.
Moreover the analog signals are more prone to noise because of there low voltage swing..........generally we have adc's with input range around 500mv.
this low swing can be easily effected by the substrate noise, i.e. transistor's noise itself.
thus we need to take a lot of care for the analog signals, like matching, shielding, etc.
there are much more issues which you can find in books.....
hope this gives you a basic idea............