Currently JIRA runs each post-function of a transition unconditionally. The only way to provide conditions is to define multiple transitions separated by conditions. This would lead to an exponentially growing number of alternate transitions as number of post-functions (and conditions) grow.

It would be useful if we could define condition(s) for individual post-functions of a single transition. Something like:
1-Run post-function A
2-Run post-function B if user is in group X.
3-Run post-function C if (mycustomfield = Y and user is in role Z)

There are some add-ons in the market that introduce post-functions that have conditions. But those conditions apply only to those specific post-functions. If JIRA supports conditions then all post-functions, both system and add-on, can become conditional.

The same mechanism and library for transition conditions can be used. This way any new transition condition functions introduced by an add-on could also be used for post-function conditions.

I don't know the inner workings of JIRA of course but I believe this could be implemented without breaking anything else and would give much more power and flexibility to the already powerful JIRA workflow engine.