When I back up from one hard drive to another with Ghost I can switch those hard drives at will and the computer doesn't seem to mind. When I back up HDD A onto HDD B with dd, HDD B performs slower and blue screens all the time (windows). What is Ghost doing that dd isn't?
Ghost actually understands the file system (FAT, NTFS, etc) and just copies the files. It can intelligently avoid free spaces in the partition and can apply compression to save space/speed up transfer.
When the files are restored, they are not put to their original location, but just placed consecutively (for non-special files) as if they're newly created. That's why sometimes a Ghost image restore is the fastest way to defragment a large partition.
dd just sees the whole partition as a giant binary file and copies it verbatim. It will copy all the deleted files and free spaces. It has no built-in compression neither. Thus it is generally slower and unrecommended for large/empty partitions.