Here's what I've noticed after working in Java pretty much forever (1996-present) and being forced to work in .NET now (2010). When you compare Java language to C#, and the JVM to the CLR, I think it's basically a wash. Minor, albeit annoying, syntactical differences, different flavors of syntax sugar, etc. Not much to talk about there.
Next up is something that's a difference, but not huge: the standard libraries available on each platform. Java collections, IMHO, are superior to those in .NET. LINQ does offer something new to .NET and is nonexistent in pure Java, but with the advent of other programming languages for the JVM, like Groovy & Scala, those holes have been plugged, which brings me to my main point.
What really separates Java & .NET is the ecosystem of projects & languages available. Java, thanks to its low (free) cost of entry, has enjoyed tremendous innovation over the years. From JUnit to today's BDD frameworks, to the persistence frameworks like JDO & iBatis, to dependency injection, IoC and AOP, to the new NoSQL technologies, to build & dependency management tools like Ant, Ivy, Maven & Gradle, to OSGi, and to open source libraries too numerous to count that can do just about anything low level that you need done, Java's ecosystem simply can't be beat. The level of innovation on the Java platform is simply staggering.
Having said all that, Java is not without its warts. EJB 1 & 2 were terrible, especially entity beans. It wasn't until much later that all of enterprise Java took a lesson from the playbook of SpringSource, who revolutionized Java development. JDO provided a great, universal, lightweight persistence model that is still going strong today, but the JCP, due to its political nature, felt the need to reinvent the wheel (and do it worse) with exclusively-relational JPA. Logging in Java was being served fine by log4j, but then the JCP felt the need to change all that and ignore the de facto standard. The result was Apache's commons-logging, so that you could use either logging technology and continue coding. We'll see where modules and superpackages versus OSGi end up, and where closures are going. It wasn't looking good for a while, but now the JCP might be learning some lessons from the past.
Anyway, while Java is not perfect, it has stimulated just amazing work. The support of other languages on the platform make for a promising future, too. I've always walked away from an experience with Groovy & Grails saying "wow". I'm very interested in the promise of Scala, too. .NET is showing signs of innovation life in things like its dynamic type and Expando object (taken from Groovy, BTW), but it takes the whole community to really produce useful stuff. Codeplex is a start, but is absolutely dwarfed by the numerous pure Java or JVM-compliant projects out there in open source land.
Only time will tell if the .NET ecosystem can ever catch up with that of Java.
