Not fully designed in my mind but a very handy when condition would be to run based on the build cause. 

For example to run lengthy performance tests only when triggered by a timer or auto deploy to staging when run by a specific user/role.

Example:

stage {
    when {
        triggeredBy timer
    }
}

stage {
    when {
        triggeredBy user
    }
}

stage {
    when {
        triggeredBy user('bobby')
    }
}

Some more trigger specific parameters might be needed so probably an extension point is needed for this that plugin authors can implement:

stage {
    when {
        triggeredBy gerrit(event: patchSetCreated, project: 'platform/my-module')
    }
}

But a lower level plain "class comparer" might solve most general cases until a more specific extension point is implemented by a plugin author

stage {
    when {
        triggeredBy cause($class: 'DockerRegistryWebHookCause', 'pushNotification.repoName': 'acme/my-app')
    }
}

Though getting the syntax right will be tricky, so snippetizer support would be good for that.