The thing that worries me about hit/expertise is the lack of mitigation stats that are available on these items as well. For the most part (from a feral tank perspective) I avoid hit and expertise like the plague because it offers very little to no advantages for me.
I currently run 10mans in which I am VERY constantly asked to kick for our runs to be successful. I love that, I love not only having to cycle cooldowns and maintain threat/make raid calls and positioning adjustments, but be able to go from a 'good tank' to a 'great tank'. The misses on kicks in P2 Nefarian make me hate that, but with the upcoming fix to interrupts that should be taken care of.
My concern is the additional stat stacking that tanks already have too much of. As a feral we need a lot of stam, agil, dodge, mastery. That is 4 stats to stack (not mentioning armor because its tier capped anyways). On my lock, I only need hit, haste and int. Yea parts of crit/mastery can help, but they are not essential. If you make hit/expertise necessary for tanks (again just from a feral standpoint) we will need to have 6 different stats to stack. namely stam (more hps and higher vengeance stack for more self heals and SD procs), agil (dodge, and higher vengeance stack), crit(SD procs only on crit), mastery(higher SD cap) expertise and hit.
To me 6 stats is a little far fetched for most tanks that play this game... I already see the average tank as sub par due to the LFG tool and having to deal with slow/bad tanks. Let alone requiring them to learn how to gem/gear correctly. It will be a designer nightmare that in my opinion would be better left alone