Linux has SDL for DirectX-like functionality. It works pretty well, and some commercial developers have used it in place of DirectX on Windows games (Neverwinter Nights?)
Linux DOES have 3D driver issues. Downloading and installing either ATI or nVidia's binary-only package is a PITA for anyone who isn't a Linux geek to begin with.
However, I don't think that's the thing that keeps people off Linux. Realistically, only a small minority of users want to play high-end 3d games.
I don't think it's Office, either. Macs have Office (but not Outlook or Access), and they're probably even less common in general business settings than Linux.
I think the problem is that the training and support costs for switching to something else would go through the roof in the short term, and that's all most businesses can see.
Balance training and support for Windows vs. the ridiculous hardware requirements for Vista, though, and in the near term I think some gains might be made.
I showed Vista to a customer who had planned to skip from 2000 to "whatever is after XP" and explained that even his newest PCs wouldn't meet Vista's minimum requirements. The first words out of his mouth were "What are my other options?"
