You are not specifying the device to mount, only the directory so it is telling you that you are missing one of the required parameters.
I believe that the
busybox mount will let you get away with not supplying the device, if it can figure out the device to mount, but the "stock"
mount requires that you supply both the directory and the device.
you would have to do the following:
# mount -o remount,rw /dev/block/mtdblock4 /system
or you could try the
# busybox mount -o remount,rw /system
but you should always supply the device to mount, so there is no confusion or issue with mounting the wrong device.