2) great third party libraries,
3) decent documentation and tons of books and instructional material,
4) best-in-class virtual machine, good GC, as speedy as you get with managed code,
5) tons of profilers and development tools
6) several industry leading tools written in it (Hadoop, Lucene/Solr, Hbase, etc) and for it (IDEA, Eclipse),
7) Enterprise support from big companies (Sun, now Oracle) and IBM, and several smaller ones
8) Keeps improving (closures added in current 8 beta for example)
9) A large ecosystem of interoperating JVM languages, from Ruby/Python like (Groovy) to Haskell like (Scala), to Lisps (Clojure)
10) 15 year history, and at some point it fixed a lot of C++ pain points for enterprise programers