If the major "enterprise" software vendors would make their "web-based" applications work in Firefox most of the problem would be solved. I write "web-based" because many of the "enterprise" "web-based" applications are really just a Java applet or ActiveX control running in a browser. And yes, I realize that Java applets should work in Firefox but some of the "enterprise" vendors managed to mess that up.

Office and an Exchange client (Entourage) already exist for the Mac. Macs can connect to Windows file shares. There are RDP clients for the Mac. VMware exists for the Mac (and works quite well).

What is missing?

I use a mac (officially sanctioned) at work. There are only a few times I envy my PC using colleagues and that's the functionality they have with Outlook. Nothing on my Mac gives me that kind of functionality. Sure I can hack up alternatives using a mix of apps, but it's not the same experience.

Why not just continue the good software that works theme, it seems enterprise is always code for big and bloated

For home and small business users, everything does just work and no one is stopping you from buying a Mac. But getting a Mac at a large company is oftentimes out of the question because nothing interoperates. The stuff they're using now _needs_ Windows or IE, so you either gotta port, move to the web, or virtualize.