[Verizon Samsung Galaxy S 4] kd-:
So I ended up giving up on waiting for the boot animation - turns out TWRP couldn't mount /data afterwards, which wasn't entirely surprising. I ended up having to go with the nuclear option of wiping the "internal storage" partition (luckily had the foresight to move everything to my SD card before trying this).

Strangely enough, it seemed to work after I ran the encryption again without any setup and without actually setting a PIN - I think the phone only did a soft reboot though. After a power-off and power-on again it seems like /data can't actually be mounted, since its a 256MB tmpfs:

shell@jgedlte:/ $ df
Filesystem Size Used Free Blksize
/dev 904.1M 60.0K 904.1M 4096
/sys/fs/cgroup 904.1M 12.0K 904.1M 4096
/mnt/asec 904.1M 0.0K 904.1M 4096
/mnt/obb 904.1M 0.0K 904.1M 4096
/system 2.7G 613.1M 2.1G 4096
[B]/data 256.0M 83.0M 173.0M 4096[/B]
/cache 2.0G 35.6M 2.0G 4096
/firmware 86.0M 8.8M 77.2M 16384
/firmware-mdm 86.0M 49.8M 36.2M 16384
/efs 13.4M 4.2M 9.2M 4096
/mnt/media_rw/extSdCard: Permission denied
/storage/extSdCard 29.7G 5.1G 24.6G 32768

shell@jgedlte:/ $ mount
rootfs / rootfs ro,relatime 0 0
tmpfs /dev tmpfs rw,seclabel,nosuid,relatime,mode=755 0 0
devpts /dev/pts devpts rw,seclabel,relatime,mode=600 0 0
proc /proc proc rw,relatime 0 0
sysfs /sys sysfs rw,seclabel,relatime 0 0
selinuxfs /sys/fs/selinux selinuxfs rw,relatime 0 0
debugfs /sys/kernel/debug debugfs rw,relatime 0 0
none /acct cgroup rw,relatime,cpuacct 0 0
none /sys/fs/cgroup tmpfs rw,seclabel,relatime,mode=750,gid=1000 0 0
tmpfs /mnt/asec tmpfs rw,seclabel,relatime,mode=755,gid=1000 0 0
tmpfs /mnt/obb tmpfs rw,seclabel,relatime,mode=755,gid=1000 0 0
none /dev/cpuctl cgroup rw,relatime,cpu 0 0
/dev/block/platform/msm_sdcc.1/by-name/system /system ext4 ro,seclabel,relatime,data=ordered 0 0
[B]tmpfs /data tmpfs rw,seclabel,nosuid,nodev,noatime,size=262144k,mode=771,uid=1000,gid=1000 0 0[/B]
/dev/block/platform/msm_sdcc.1/by-name/cache /cache ext4 rw,seclabel,nosuid,nodev,noatime,discard,journal_checksum,journal_async_commit,noauto_da_alloc,errors=panic,data=ordered 0 0
/dev/block/platform/msm_sdcc.1/by-name/apnhlos /firmware vfat ro,relatime,uid=1000,gid=1000,fmask=0337,dmask=0227,codepage=cp437,iocharset=iso8859-1,shortname=lower,errors=remount-ro 0 0
/dev/block/platform/msm_sdcc.1/by-name/mdm /firmware-mdm vfat ro,relatime,uid=1000,gid=1000,fmask=0337,dmask=0227,codepage=cp437,iocharset=iso8859-1,shortname=lower,errors=remount-ro 0 0
/dev/block/platform/msm_sdcc.1/by-name/efs /efs ext4 rw,seclabel,nosuid,nodev,noatime,discard,journal_checksum,journal_async_commit,noauto_da_alloc,errors=panic,data=ordered 0 0
/dev/block/vold/179:33 /mnt/media_rw/extSdCard exfat rw,dirsync,nosuid,nodev,noexec,noatime,nodiratime,uid=1023,gid=1023,fmask=0007,dmask=0007,allow_utime=0020,codepage=cp437,iocharset=utf8,namecase=0,errors=remount-ro 0 0
/dev/fuse /storage/extSdCard fuse rw,nosuid,nodev,relatime,user_id=1023,group_id=1023,default_permissions,allow_other 0 0

I tried getting a logcat in this state from boot but...

shell@jgedlte:/ $ logcat
logcat read failure: No such file or directory

And I couldn't get root rights either to read other diagnostic info:

shell@jgedlte:/ $ su
1|shell@jgedlte:/ $ cat /proc/kmsg
cat /proc/kmsg
/system/bin/sh: cat: /proc/kmsg: Permission denied
1|shell@jgedlte:/ $ dmesg
dmesg
klogctl: Permission denied
1|shell@jgedlte:/ $

I'm assuming those "1"s are exit codes indicating failure.

I attached a logcat of the encryption process, if that helps - from all the "Cryptfs" entries it looks like things worked.

EDIT: also attached a dmesg from TWRP, but I'm not sure TWRP even supports encryption...

Perhaps this is an issue with the kernel? Hope we can get this working - if someone needs more info let me know.