2. Plenty of code samples and live examples. Code samples are a great way to get your own map up and running quickly without having to figure everything out first. While the samples are not perfect they are a good way to discover map functionality that you might want to incorporate.
3. Support for a ton of layer and data formats. From GeoRSS and GeoJSON to KML and shapefile, this API pretty much covers it.
4. Well written API reference documentation. If you are going to use an API (any API) you want to be able to understand it so you can use it effectively. Good documentation is critical for this. The Esri JS API is (in my own humble opinion) extremely easy to read and understand. Each class has all the information you need to use it without having to decode any meanings. Furthermore, there are links from each class to samples that use it.
5. Editing tools for online editing of SDE data. Let’s be honest, online editing through a web browser is not a great idea if you are trying to build accurate data. However, there is definitely a place for it. If you are trying to outline an area inhabited by a herd of elk or mark an area of weed infestation, browser editing is probably fine. It is great just to be able to have this ability so more diverse mapping applications can be developed.