What makes the Flickr API not RESTful?

Question

I'm about to start developing a REST API for a client's website and I've been doing some research. I came across this useful question on the gold standard for APIs.

Prior to reading this post I'd thought about using the Flickr API as a point of reference. However this comment on the above question made me think twice:

The Flickr API is particularly hilarious, for example. It advertises itself as RESTful and yet is nothing of the sort! – NathanE

I'm particularly interested in what makes the Flickr API not RESTful and what affect these none RESTful elements have.

Answer 1

• Incorrect use of HTTP verbs: Flicker uses the verb "replace" instead of HTTP PUT in http://api.flickr.com/services/replace/. Another example: http://api.flickr.com/services/rest/?method=flickr.test.echo&name=value "method" should be one of the HTTP methods (GET in this case).
• Flicker doesn't use HTTP status codes correctly.
• Authentication is not RESTful