I think it would be useful to be able to validate the schema of work queue items before they get added to a work queue. My thought is an XSD validation string or or some such mechanism could be associated with the work queue. The Add to Queue action could use it to validate that the data being added to the queue is valid before proceeding.
The advantage of this is that processes that use the work queue would be able to safely assume that any data entered into the queue is valid. Further, it would mean that exceptions due to invalid data would happen at the time that data is being added to the queue, as opposed to when the data is being read from the queue.
On a practical level, this may not happen often since users should be testing what data is being added into queues before going live with it. However, this would still be a gain because it would be one less thing that you have to think about while building processes.