Here's a screenshot of the datatypes that we'll be able to start supporting once this patch gets merged.

In addition to the wikibase-item type ("Depicts"), I have string, quantity, and url datatypes now working. URL and String types use the new StringInputWidget, while quantity types use the QuantityInputWidget. Both of these new widgets could benefit from some light touches of design, I think. In particular, I think that some sort of button that says "add value" to the right of the input field for string and quantity inputs would be helpful. Currently the only way a user can actually input a value (after typing something into the field) is to hit the "enter" key on the keyboard.

Here's an example:

1. User inputs a Quantity value (either by typing or by clicking the plus/minus buttons):
   
2. Hitting "enter" puts the panel into edit mode, and allows user to add qualifiers:
   
3. Clicking the "publish" button works as normal, and puts the panel back into read mode:
   

Other properties (coordinates, time, etc) will need a somewhat different treatment. String and Quantity are the most basic types so this is probably where we should start. Fortunately, users won't be able to add properties with non-item datatypes until we explicitly whitelist the type in question, so we can roll out support one at a time as we come up with new UI elements.

Let me know how you'd like to handle these and I can start updating the appropriate widgets for a better UX.

(Adding Structured-Data-Backlog project tag as SDC Design / Structured-Data-Design project tags have been archived, and as tasks need to have an active project tag associated so they can be found on a project workboard. Not sure whether to also add Design.)