In a word, none.
Graphics performance and stability, especially with device sharing. Every time I've used virtual box I've had problems sharing USB devices with VMs.
I'm damned if I'm going to upgrade and I encourage everyone else to show VMWare what they think of being treated like a cash cow. If you want VMWare to treat you like Parallels and Adobe--and, truth be told, Apple--then carry on and pay them for this "upgrade". For those of us who don't want this, just vote with your wallet.
If Fusion 4 does what you want, why would you upgrade anyways? If there's something of value to you in Fusion 5, and I think there is otherwise you wouldn't post here complaining, why shouldn't VMWare and their developers get paid for creating that value?
What really annoys me is that VMWare player -- the PC version -- is free. Just like VirtualBox.
Just... say... no.
VMWare player is in no way, shape, or form equivalent to Fusion. In particular, you can't create VMs or do snapshots.