I'm a computer professional and I don't hate Macs, I support them along with PCs and linux machines. However, I think I can answer your question in detail.
Macs are frustrating to fix. As an IT person, the lack of good information for how to fix Macs is a huge minus for me and my organization.
Macs broadcast like crazy.
Macs have a problem printing to network-shared printers.
Macs have no WINS.
You can't join a mac to a domain without pulling teeth (or all of your hair out).
Old Mac files are two-fold with data and executable sections separate - this makes them extremely breakable in file server type situations. SMB and/or Samba is a bear to configure correctly.
Mac users are less well-informed about basic things such as DNS, networking, and file management, among other things. Because Macs are so "user friendly" they hide their powerful features behind a nice interface. Therefore, communicating with a Mac user is like communicating with a grandma who is learning about computers for the first time.
In a business environment, Macs are limited in functionality, for example, Outlook on a mac will not perform many advanced features. This is extremely frustrating for both the Mac user and the IT person. Usually the IT person gets blamed for not being able to make it work. But it's not their fault that the software simply will not perform advanced tasks, such as reading a Global Address Book, without a great deal of effort.
IT people aren't well informed about Macs. Macs need a great deal of free software to run correctly. IT organizations fear free software for obvious reasons of no-support, and virus risk. So they keep telling Mac users to not use what is essential to their daily functioning. This breaks down corporate communications because the Mac users will simply stop talking to the IT people if they feel "ratted out" to management.
On the plus side: Macs have Fink, which makes a Mac more like a Linux machine - that's computer geek paradise. A stable machine that has a nice interface and a huge amount of cool software that's also useful for programmers and scientists.
But there's even a downside to that - in a business environment, you don't want a bunch of freeware that you don't have good support for. It's too risky. If you lose the one or two IT people who can maintain it, and understand it, then you're usually totally screwed because the Mac user can't be trusted to know how it works.
I once gave a presentation to the Mac graphics group at work. They were totally fascinated by my explanation of how their computer problems were related to their not being joined to the corporate domain, and broadcasting limitations on the network. It was all news to them.
However, they still did not give us permission to join their computers to the domain and use the advanced Mac OS X features instead of broadcasting. Why? Because they didn't trust any changes. There had been a track record of hostility between them and the IT dept, so they almost never accepted our help. You can see how this will lead to bad feelings all around.
Don't get me wrong. I love Macs and I wish I could support them better, but right now, for the many reasons I've given above, it's not possible.