A lot of frustration at times. 
The Java language itself is pretty easy to pick up, especially if you have used C/C++ before. The problem is the large number of libraries to sift through. There is a lot there, which is a good thing, but it does require some learning. Plus, the Java IDEs are real memory hogs. But then, the Xojo IDE can be a resource hog too, especially if you load all the MBS plugins.
One thing I really like is that if I have question/problem, I can google it and usually find an answer within seconds. The amount of Java resources out there is huge.
Also, a lot more 3rd party libraries and components to choose from. One of my favorites is Jexer, which is. text mode user interface library (think Turbo Vision, if you are old enough). Not real practical, but it is neat and I’ve been playing with it. I’m a hobbyist so I do a lot of tinkering.