d - MrsMopar thank you! 

D - GazetaBitcoin

I'd say that credibility / trustworthyness is a must. The people running the mixer should do their best for eearning users' trust.

Then a flawless functionality is also needed. All options offered by the website should work as designed. This will cerrainly please users.

Last, but not least: fast and reliable support team. Users always complain or have questions. Someone should always be there for them. And this also helps in earning credibility.

Note: I chose "d" since the earlier user which chose "d" had the post rejected.