Hit level 71 and lost 10 (480) points in defense. I was wearing exactly the same gear I was before I leveled and was at 490. Anyone else experience something similar?

I've switched stuff around trying to determine if it was a piece of rep gear that might have caused the problem but have yet to identify what it could be.

And to answer the question, no I do not have any of that 'loses effect after 70' stuff.
I'm also currently experiencing the same problem. I've dropped about 15 points so far. I had 500 when I was 70, then it dropped to 493 at 71, and now at 72 I am at 485 which is below the uncrittable cap. I'm not sure if it has affected me when I'm tanking, but I'll make sure to see if it's just a bug with the ui or if it's really like that. I also don't have any addons running. 
Im having a problem with defense as well, but different than yours. Mine involves gear only adding 2/5 of the +def it is supposed to.
This is not a bug. Your gear contains 'defense rating'. Defense rating (DR) converts to defense skill (the value on your character sheet) at a conversion rate that varies by level. For a level 60 : 1.5 DR = 1 defense skill. For a level 70 : 2.36 DR = 2 41/112 DR = 1 defense skill. For a level 80 : 5.4 DR = 1 defense skill.

Therefore, as you level you'll lose defense skill with the same gear because the conversion from defense rating to defense skill changes every time you level.

you could get an outstanding wow power leveling service here.