Android Studio. Number one most important thing in whole Android development story is Android Studio built on top of IntelliJ IDEA engine. Excellent off the box Gradle support, intuitive source control, intelligent code completion, responsiveness of the UI, debugging -- all top notch. They even help you complete repetitive strings in JavaDoc.
Responsiveness. Very simple and super-flexible means to layout interfaces for any platform, and means to pick the right resources (strings, values, drawables etc) for each. If I am to give just one suggestion here, it will be -- forget the visual UI designer. Give manual XML editing a try. It is very easy and pays of million times with precision and clarity in interfaces.
Building blocks. Extensive collection of modules and classes makes development a real pleasure. Take a look at AsyncTask and IntentService. These are the real pleasure to work with. Database management tooks, and speech recognition / synthesis are super handy too.
Documentation. Detailed tutorials and reference pages. In this aspect Google is light years ahead of Apple. The way Android Tutorials are designed and written is absolutely fantastic. You get just the necessary zoom level into the technology. If you need more, you drill down, if you need less -- you get up the hierarchy. In contrast, Apple has traditional all-in-one-book PDF-like papers, which makes it a complete waste of time if you need just a tad for solving the present task.
Fragments. With the introduction of fragments the development for Android went to a totally new level. They are those "sub-activities" you can embed in your views, replace, and transit to and from. You can juggle with them and use to compose interfaces for different platforms and form factors (think master-detail laid out sequentially for handhelds and as a sidebar+main area for the tablets.)
There's plenty more. I've highlighted something that stuck in memory after the recent week-long project.