I agree that LoF as a reaction is ridiculous, especially without a meaningful penalty for not being proficient with armour and shield.
You're right, I never really thought of how meaningless the penalties for not being profecient with shields and armour are. Really the only meaningful downsides are rolling with disadvantage on initiative (but my Cleric rarely likes to go first anyway) and on Dex saving throws (which are far more common than Str ones, but if that's the biggest downside I'll take my chances).
When you compare that to the AC and survivability gain you get, why wouldn't you strap every cleric into the best kind of armour they can get and screw the proficiency?
Because of this, which I had a nagging feeling I'd read, but couldn't find before (121712 How to Play) which wasn't in the previous packet:
Casting in Armour
Because of the mental focus and precise gestures required for spellcasting, you must be proficient with the armour you are wearing to cast a spell. You are otherwise too distracted and physically hampered by your armour for spellcasting.
So, you can wear whatever armour you like, but can't cast spells. Kind of a big drawback after all for a spellcaster.
Which means I now have to offer my Lightbringer the option to a) keep his deity and take off his armour, or b) switch to a deity that lets him wear armour. Fortunately he's just done something I consider fairly evil, so forcing the issue immediately can be a perfectly plausible roleplaying opportunity.
I do wish they'd stop screwing up the pre-gen characters (which are always my starting point when converting PCs from packet to packet) so this wouldn't occur. And changing a cleric from one that can wear anything to one that can suddenly wear nothing is a pretty big change!)