Well, the iMac's hard drive is SATA, while almost all FireWire enclosures are ATA. SATA FireWire enclosures do exist, but you have to search far and wide for them. (Googling "SATA firewire enclosure" brings up several vendors, none of which I have any experience with.)
As for ease of removal, if you're remotely "handy" with gagdetry, it's easy in an iMac.
tooki