I am creating new custom objects via the create() method of the REST API successfully, but then I checked "Do not allow duplicate values in this field." for a userid field. I get an error when trying to create a duplicate from the UI, but I was able to create new user with a duplicate userid via the API. This was not expected. Bug? (Aside and not important to me because I am not a REST-Purist, but "create" is a verb like a method call, not a noun like an Object, so the API seems perhaps more like XMLRPC.)
This will be fixed in the next release, thanks for noticing.