You can target characters seperately from the unit they have joined ONLY if the character (including his mount) has a unit strength of 5 or more.  This means, among other things, that characters riding steeds will be able to hide inside infantry units!

And just checking on the one major problem with that... Look Out Sir will function! The only restriction is that a character with a unit strength of 5 or more cannot get a Look Out Sir!

Great news for EVERYONE except dwarfs and skaven   In fact there are very few reasons I can think of not to put all qualifying characters on mounts.  I suppose if you intend to join skirmishing units or be on your own (dangerous now), or if they are using weapon combinations with seperate infantry and cavalry rules such as two hand weapons, hand weapon and shield, or a greatweapon. So for most wizards or characters with magic weapons, MOUNT UP!

This might be especially helpful to armies like the Tomb Kings who will be able to effectively double the move rate of the Priests who can never march.  I can also picture a few fighty characters threatening a long charge while hanging out in a unit of infantry for protection.  Fun, fun.

Please anyone tell me if I've overlooked something.