Your comparing two very different platforms... mush like a Nintendo and a Playstation2. Mac have there own set of applications... However a Mac it can access PC documents with the help of Virtual PC.. but don't expect to play your latest PC game through it.
the iMac is a consummer, non professional PowerMac.
Since most home users won't need access to additional cards for say D1 encoding, BetaCam editing ect.
Apple have designed the system to be very appearling they put in a super fast 64bit CPU, given you the state of the art TFT screen and a hold host of other features... such as Fireware... ok so no USB2, but Firewire is tons faster.
Hope this helps.
LAdesign