It's too damn slow and chews up way too much memory unless you're writing a single, long-running, multithreaded process. So, it forces you to write all applications as single, long-running, multithreaded processes.

Memory-mapped shared libraries? No. Copy-on-write fork()? No. Memory protection? Basically, no. Can be reasonably used to implement ls(1)? No.

Another reason why I hate Java is that there are going to be at least 3 people who reply to this saying that Java isn't slow.