Several people have pointed out to me that one can simply create a single encrypted “physical volume” with the Debian installer and place “logical volumes” for the various filesystems in there. [...] This method is much cleaner and to be preferred.
LVM does not support barriers. Your original setup with traditional partitions is therefore more secure with respect to power failures. See http://lwn.net/Articles/283161/
Are you sure dm-crypt supports barriers?