I love developer friendly APIs.
I want to model ours after a great one. What are some of your favorites?
It has the most comprehensive and intuitive documentation of all the APIs I've used. Their format for API documentation should become a standard.
Of course, now that they're with Facebook the developer support will likely not be as good, but the API is great as it is regardless.
Really nicely documented.
So many small companies put out giant behemoth APIs with crazy security hoops because that's what Amazon does and Amazon's stuff is really cool. As a result, nobody outside the dev team can figure out how to authenticate, so nobody actually uses the API.
Check out Twiddla's API (http://www.twiddla.com/API/) for an example of how to keep things lightweight. 2 POST endpoints and an IFRAME syntax. Only the bare minimum security, and a huge emphasis on getting your thing up and running without having to sweat the API.
Typically he talks about APIs more in the sense of library design than web service design, but he's very relevant nontheless.
Furthermore, try to keep all of your API libraries up-to-date. For example, I was almost driven crazy by Google's v1.0 Python Spreadsheet library's lack of support for insertion based on case-sensitive row names (See http://code.google.com/apis/spreadsheets/data/1.0/developers...). There is no documentation regarding the requirement that the row name must contain only lower case characters. The .NET library is v3.0 and appears to offer much greater support for this type of basic functionality.
Tumblr -- it's simple, it's clean, it returns a Tumblelog as XML over an HTTP request. You can also write posts over HTTP as well.
Provides a lot of functionality.
Basically anything you can do on the website you can do through the API.
The names of the calls are intuitive.
Provides different formats.
And most importantly has libraries available for different languages that will make the calls for you.
If you're writing a RESTful API try using webmachine. At the very least, you'll feel like you're writing the best API ever.
*NOTE: I have no relation to webmachine, just really impressed with the elegance how it is written and the API's it help produce.
If things change without warning, that is bad.
If the site is down often, that is bad.
While users are awesome, and it is great to have a developer community, depending too much on them to do your QA is asking a lot.
It's a delightfully clean RESTful JSON API.
On the other side of the coin, Google's GData APIs (e.g., Calendar, YouTube), Paypal and Facebook are cumbersome IMHO.