Oh, well, while I have no experience of maintaining an OS X system, it's painless and mostly automated on windows, so that's really no major concern.
It depends. I suppose Windows isn't too hard to keep updated if you use Windows Update. But if you need things like OpenGL drivers or something WU doesn't provide, you need to go hunting around. Not hard, but not necessary, either.
Hard drive space may be a valid concern - but it's 2011, why are we still worrying about hard drive space?
Other people may feel different, but I don't really like having wasted HD space. I myself use a 150 GB drive, or there-abouts, and as long as I keep unused apps and files cleaned off, I have no problems with space (despite having hobbies that like to use up disk space). I may need more space eventually, but as it is right now, I'm fine. Compare that to people who keep "running out" and need 1+TB drives...
Assuming OS X is your general-use system and windows is for gaming, the limitations of not being able to run applications on the other system is unimportant
I don't know about that. Personally, I leave my IM, IRC, and email clients on while playing games... mainly just in case someone needs to contact me with something important, but apparently some people like to actively chat while they play, too. If you're modding, you may also want quick access to a web browser, paint program, modeller, and whatever else (I realize the Creation Kit itself wouldn't likely be ported, but that's where Wine would help; having a native port of the game would still be a great boon even if the CK needs "help", as it were).
and for non-gaming uses a VM may be a better solution, and that *will* allow you to run both OS' applications at once. A quick google shows W7HP at £65
I thought only the more expensive, non-Home versions were allowed to be used in a VM?