Don't forget to offer PLENTY of immediately usable examples of the API. Like Pivotaltracker has copy/paste-able curl commands to try out the API. Have libs with consistent examples for all popular (and, if for some free PR, less popular languages). Version your API in the URL; you might need large changes to existing stuff, so you cannot trust on backward compatibility all the time while it is really annoying for devs when something works if they have to change it (or it stops working).
Edit; hire 'average' programmers from oDesk etc to build 'stuff' against it 'for clients' (threat it as a serious project) to figure out how easy it REALLY is to make things with it. Average programmers are, well, the average, so they will be the most likely consumers of your API. If they have no clue at all, you didn't go far enough perfecting everything. For instance we hired a bunch of devs to do a Windows plugin for our monitoring system and we found out our documentation and examples were completely unusable for them. We'll have to rewrite/refactor all of that and have them re-try.
You said it. I have this feeling people tend to forget developers are users too. The "don't make me think" rule still applies even if their job is thinking.
So if you really want people to use your system (API, library, IDE, programming language, compiler...) you should run usability tests, write easy-to follow documentation with examples and answers to common problems, make it super easy to download (if it's software), maybe make a web page where you can try it online (that helps a lot), have a nice, smooth user interface, make it behave similar to something users have probably already used, have a good name, a nice logo, a nice-looking webpage, take care of its search-engine positioning, offer services such as forums, maybe advertise a bit, sell shirts, give a free night at a spa to one of the users every month (hey why not)... wow that list came longer than expected. Well you get the point.
Edit; hire 'average' programmers from oDesk etc to build 'stuff' against it 'for clients' (threat it as a serious project) to figure out how easy it REALLY is to make things with it. Average programmers are, well, the average, so they will be the most likely consumers of your API. If they have no clue at all, you didn't go far enough perfecting everything. For instance we hired a bunch of devs to do a Windows plugin for our monitoring system and we found out our documentation and examples were completely unusable for them. We'll have to rewrite/refactor all of that and have them re-try.