What makes an API great?

After few years of Java programming, I improved slower than I expected but still... When designing complex software which needs to be reused, I still find it difficult to make quick and "good" choices without adding too much complexity.

In the last branch of XPontus XML Editor(major refractoring), I had a dilemma:

• Release soon : couple of patches and dirty classes which do the job
  
• Wait 4 or 5 months to be ready : clean up the code, perform thorough testing, remove unused classes, add or redesign interfaces, etc.

I choose to release early and fix what I could see or had time to fix. One month after the release, I was like "why did I do it that way? what is that useless piece of code doing over here, etc."

IMHO, an API is great when it's

• simple
• useful
• flexible(without adding too much complexity when it's unnecessary).

I tend to prefer APIs which expose few interfaces, abstract classes and some concrete classes. Complex applications APIs without/with few interfaces, are somehow difficult to maintain, refractor, make evolve.