I honestly think the biggest hurdle for many C# developers trying to learn Java is learning a new IDE. Visual Studio is great, and when you're coding in C# for a long period of time, you get used to it. When having to move over to Eclipse or Netbeans, you suddenly feel lost. How do I set a breakpoint? Where's the immediate window? How do I create a windows app? etc etc... I know this sounds crazy, but I'm telling you, people get very attached to their IDE's and have a tough time getting used to new ones...
