What are some of your favorite APIs? Why? Is it docs? Feature set? What draws you to the next big API?

I'm probably biased, but I actually think the flow we built at developers.dwolla.com is pretty cool. The intent was to have content that's relevant to both developers, and biz people...

It's still a working progress, but I think that ideally, the best thing you can do for developers, is give them the ability to try out your API right there and then, without requiring them to do too much (or at all) signup work...

I may have also misread the question :) Oops....

Fav API right now: IronIO's Workers and Queue ... Docs are slightly confusing still... But, the product itself is so much fun. I'm using their cloud CRON jobs for a bunch of daily routines, and their queues in almost every app I build now.

Their free tier is more than enough for me, and they promise to always keep the free tier around. Ends up costing far less than my EC2 instance (I'm already past the 1 free year mark)...

I'm a huge fan of the foursquare API. I think they do an awesome job in their docs of organizing and showing a lot of endpoints:
developer.foursquare.com

I've been using the Twilio API for a long time and it's still one of my favorites. I think they have the best account dashboard of any API I've used. The debugger alone has saved me countless hours of sorting out issues in my code:
twilio.com

What draws me most to an API initially is "Time To First Hello World" (TTFHW). If I'm not able to quickly build something interesting I've lost interest. TTFHW tends to be a result of both ease of use of the API and good documentation.

Stripe has very nice docs, if they'd only get them to 100% coverage. They're at about 95% right now, so it's frustrating when I come across a case not covered in the docs, just because it's so out of place.

Pusher has some pretty nice docs, as well. And Twilio's almost go without saying, at this point. Everyone loves those. I love Google's generated client libs, but their docs are an iffy proposition, at best.

Docs come down to two things for me: how long it takes me to get started, and how many times I need information not found in the docs.

On a side note: I hate when docs say you can't do something, but don't say what happens when you do it anyways.

Gah, I totally changed the question in my head.

I like an API if it has a strong, flexible toolset I can fit into my workflow, and if it has enough docs that I never need to ask someone in a support channel what the behaviour is when I do something stupid. I have a tendency to do stupid, unexpected things.

These are all great answers to the question :) Don't feel bad about twisting it too much :P

I think there's something to be said about over-documentation too..

Well organized documents + examples go a long way. I guess that can be attributed to UI/UX...

I really like the webhook paradigm. Foursquare push API, Twilio, IFTTT, etc. I'm also a big fan of APIs that do real world stuff for you. I'm looking forward to the Pebble API as well, which will give us API endpoints for a wristwatch.