Minimal example payload for user creation using Force.com REST API. Post this to https://instance.salesforce.com/services/data/v27.0/sobjects/User

user.json

{
  "Username" : "user@example.com",
	"Alias" : "user",
	"Email" : "me@example.com",
	"FirstName" : "Test",
	"LastName" : "User",
	"ProfileId" : "PROFILE_RECORD_ID",
	"LanguageLocaleKey" : "en_US",
	"LocaleSidKey" : "en_US",
	"EmailEncodingKey" : "UTF-8",
	"TimeZoneSidKey" : "America/Los_Angeles"
}