What languages would be a good replacement for Java?
C# is the only thing that will meet your needs and not feel hopelessly archaic, or frustrate with limited library. For open source/non-windows, use mono. It's a good, mature implementation of most of what's important in the CLR. 
Some things (WPF, WCF, etc) are "missing" from mono, but these aren't so much part of the platform as they are windows-specific proprietary toolkits. Some of them are being implemented slowly in mono, some aren't. Coming from java you won't miss them because you're looking for a platform and good standard libraries to build upon, not a gui toolkit or whiz-bang communication framework.
As far as a platform to build stuff with that's "like" java and offers similar levels of functionality, C# + CLR is the clearest option.
I may be posting a premature question, and maybe I'm just freaking out for no reason, but the way Oracle is handling Java is not very promising. I am a nerd who fell in love with Java from the first sight, and use it all the time in my personal/freelance projects but now I am thinking of a replacement.
I am fluent in C#/VB.NET too but I am looking for something more like:

Open Source
Compiled
Cross-Platform
Object Oriented
Large standard library
Extensive documentation
Web development is a major plus

I was thinking about a compromise: Python/Django for web development (or PHP), and Qt for thick client development. Anyone with better thoughts?