General:
* Distinction
* Disparity
* Variance
* Variation
* Discrepancy
* Divergence
* Dissimilarity
* Contrast
* Disagreement
More specific:
* For opinions: divergence, disagreement, dissension
* For amounts: discrepancy, disparity, variation
* For appearance: contrast, dissimilarity
* For something that sets things apart: distinction, hallmark, characteristic
The best synonym will depend on the specific situation and the nuance you want to convey.