With compliance and audit (SAS-70) seperation of duties is a major issue. In a larger company I suppose all users could be broken into distinct groups with distinct responsibilities, but for smaller shops this isn't possible and thus enforcing proper seperation of duties isn't really possible currently (that I have been able to see).

Issue JRA-12660 would be a good start, but more advanced options would be ideal.

It would be great to have workflow conditions where you could say a person can only do transition B if they DIDN'T do transition A.

Simple example would be if a workflow requires someone to say the issue is fixed, and then someone else at a later point must say they have tested the fix (user acceptance testing). For seperation of duties reasons, no matter the persons security group, etc they should not be allowed to both make the fix and also verify it via user acceptance testing.