I am building a Visual workflow where one of the fields is a drop down for the user to select a country. Is there a way to bring the list of all the countries in the world via Dynamic Choice? or do I need to create the whole list manually?
You'll want to take a look at Implementing State and Country Picklists which allows you to do what you're asking. However, it can also have implications on your entire org if you choose to implement the feature; all of which is explained in the document.
As far as I am aware the dynamic choice option can only query an object not meta data. You could have a custom object called country with a record for each country, then query the object. If it's a one off it may be easier to manually recreate the picklist in flow but if you are likely to use it in other flows or want to be able to give a non flow admin the ability to update the list the custom object route is viable.
There are number of enhancements to flow in summer 14 buy I don't think querying picklist is one of them. Would love to be wrong!