How is non-standard Cantonese spoken by some mainland Chinese speakers different from "standard" pronunciation?
Edit: Sorry that I misunderstood the question.  I thought Maroon was talking about non-native Cantonese speakers.



Most native Mandarin speakers have trouble handling the rising (上聲) and departing tones (去聲) in Cantonese.  It is because Cantonese further differentiates them into high-rising (陰上), low-rising (陽上), high-level (陰去) and low-level (陽去).  (Please refer to http://zh.wikipedia.org/wiki/%E5%9B%9B%E8%81%B2#.E7.8E.B0.E4.BB.A3.E6.96.B9.E8.A8.80.E5.A3.B0.E8.B0.83.E5.AF.B9.E7.85.A7 for the exact tone contours.)

They also have difficulties pronouncing checked tones (入聲) as they are also absent from Mandarin.  And, again, there are three levels of checked tones (陰入, 中入 and 陽入), making it even more difficult to manage.