In summary, and in order of importance (at least IMO):
1. Free
2. Crossplatform
3. Backed by a large corporation
4. It was very similar to the already well known C++, but with automatic garbage collection and fewer "quirks"
5. Large standard library
6. A number of standards (JDBC, JMS, J2EE etc.) that allow for good interoperability between libraries
7. Fast virtual machine (well, at least the latest ones are; the first versions of Java were a little sluggish)
8. Good threading support
I can only think of one way the Internet helped Java's adoption, and that's simple that distributing a free programming language is far easier online. I don't think anyone can claim that Java applets were exactly much of a success
