Let me try to answer your question one by one.
1. Java is a language that is supported on desktop, browser, mobile and embedded system. You can find java applications on web like social networking, desktop application like eclipse, and mobile apps in android. So it's helpful if you learn this language.
2. Java is object oriented language. So basic concepts involve both OOP and Procedural language. You'd benefit learning about collections, file I/O, graphics, networking, web server, JSP etc.
3. There is no most important thing. Like any other OOP based language, you have to learn the concept of the java. Also learning about JVM and JIT helps when you are learning about java.
4. Java is in the market for last 20 years and even earlier than that. So you do benefit learning concepts. Also despite other languages being more popular you're going to benefit learning java. As most of the MNC jobs have java preference, so there is that benefit too.
5. Learn testing your code. Learn how to write the standardized code. Also do learn about how to catch and display your errors. Learn from others code. Learn how to optimize the code. It takes time to learn these things.