Actually, the language itself isn't the greatest. It won over c++ developers because of 3 reasons : a standard network library, closely tied to a (reasonably) well-designed, truly OO I/O library; a standard GUI api that worked across platforms, and considerable simplicity and code-readability over full-effect-templates in C++ (read Alexandrescu's Modern C++ Design recently?).
A fourth capability, in the first released version of java, but not used to its fullest capacity until Java2, was the use of "interface" over abstract base classes as a means of building frameworks in the newer library components such as JDBC, Collections, and XML. C++ always had this, but "interface" is a much simpler syntax and means of expression verbally over "abstract base classes with only pure virtual functions" (the C++ version of a Java interface). Also, newer C++ libraries and designs tend to rely on templates and traits to enforce an interface (ala generic programming) rather than class design, because its easier to just write a class than to design a hierarchy -- java's "interface" took the hierarchy out of places it didn't need to be).
Now, one of those ended up an utter failure (AWT), and its replacement (Swing) though amazingly more successful as far as design and power goes, is as noted dog-slow (though its something that does get faster as machines get faster; moore's law does help Swing considerably). I personally love swing just because (when used properly) the WORA DOES work (layout management is the #1 problem for almost every bad interface out there, and that's not unique to swing; i recall a lot of bad motif layouts too from windows programmers not used to Xt's approach); also, the power in using renderers for complex components and dividing up responsibility of showing the look vs managing the data, is something i miss in any other gui library out there.
The second, the standard networking library tied to the I/O library, remains its brilliant point and the basis for Java's most successful libraries and projects, including all its server-side work. Bjarne is most impressed that the standard socket + stream library that works on ALL platforms (its the one that's most reliable in that respect of WORA) that he's planning to propose a standard socket interface to C++, though I think its now too little too late. I'm not saying that java.net and java.io aren't flawed. The use of abstract base classes for Socket and URLConnection, which likely dates to before the interface keyword was introduced is a "bad thing"; java.util.Dictionary was like this as well, but at least that's been deprecated out. Similarly, java.nio addresses most of java.io's problems, but at the considerable cost of code simplicity; if you need java.nio, it'll take a lot of time and work to use it correctly, and few books and articles are really making it clear when you actually need it. In the early stages of new technology, the "how do i use it" well-buries the "when do i need it" question.
The language itself has remained simple, with only 2 partially-incompatible changes over the years (inner classes including anonymous, and assert), and this may be its one saving grace against C# (which has a more complex syntax, but currently a much simpler library based on a cleaner syntax to most of MFC -- that will change in the future as M$ will always code-bloat their products).
The "interface" syntax is to me still Java's most powerful feature; again not in that it provides any more capabilities over C++'s abstraction (as i worded it above), but by being so simple, did more to improve inexperienced developer's OO code than any other OO syntax out there (IMHO). I'm not suprised at all that C# also chose to keep the interface keyword.