First, huge open source ecosystem and culture. Mature open source projects, culture of writing blogs and tutorials (that one will die due to changes in search engines, but it was super nice while it lasted).

Second, working in C# felt clunky, as if every other thing was done to check the checkbox "done" and the author called it the day once it sorta kinda worked. There was some additional syntactic sugar in that language that was nice, but it did not made that much difference in practice and I don't miss it after coming back to java.

Third, I found the obsession with bashing java by people who have no idea how java projects look like and which problems they have annoying.