Having the ability to specify conditions and validators which trigger when the transitioning issue matches a JQL statement would be extremely powerful.  

For example when transitioning issue "PROJ-1", my validator would run a JQL search on

(key = PROJ-1) AND (my custom JQL statement)

and the transition would succeed if and only if the search returned results.

Most of my team's multi-field business logic can be expressed in JQL statements.  I know this because I currently have a dashboard of JQL filter widgets which list issues which are in an invalid state.  I'd love it if tickets couldn't get into the invalid state in the first place.