2) Documentation: Code examples, easy things like: "This is what you send, this is what you get." "If you change this, now you get this." Simple example heavy documentation. 1 example is worth a hundred lines of documentation.
3) Response Time: meh. If your at or around 100ms you don't need to worry to much. If people want ~50+ calls per second, then ask them what their use case is, or have them pony up money. Then worry about response time.
4) Flexibility: Largely over rated. You can waste a ton of processing power trying to figure out what the user meant to say. Most of it is wasted, if their query doesn't make sense return an error. Your performance will go up. This hooks into documentation, if your documentation is easy enough to understand you won't have to fuzz the queries.
5) Code Samples: See documentation.