Gear-wise I am finding (mainly from EJ number crunching and a little personal experience, but I have 0 raid experience despite my gear being good enough to MT 25 mans) there's no reason not to take defense over dodge/parry rating as far as gems/enchanting goes, but like you say, block is the best rating. It's part of why warrior/pally tanks will be the best for mitigation.
The defense "diminishing returns" thing is kind of a misnomer in that it doesn't really have a stiff penalty. Basically there's just no way to stack enough defense and dodge/parry rating to hit absurd avoidance like it was possible for druids and even rogues to do in TBC. Defense is still a good rating, even when you are capped, just because it will still affect your block/parry/dodge and also your miss chance, even if miss/dodge/parry all have DRs.
Parry is in fact the worst rating in terms of point buy, which kind of hurts DKs (and is why they get STR converted to parry) but it's because of parry haste, really. So it's honestly pretty balanced, and it's not like you should pass up tank gear just because it has parry rating if it's an upgrade otherwise. Block rating is definitely the best, but it's a mitigation rating, not avoidance. Not that I don't still go for block rating anyway, since it's a titanic amount of mitigation and as a pally, I get a little more threat. obviously since you're a warrior you don't really get extra threat out of it, but yeah, it's way easier to stack block rating and get tons of block than it is to stack dodge/parry rating. And since it's mitigation, it has no DR.
for gems, I see no reason not to stack defense/stamina, str/defense, defense/expertise (if you aren't at the 6.5% soft cap), or str/stamina (though iirc this one isn't available yet from twilight opals) based upon what's best for that socket. That is, assuming you either want the socket bonus (I usually do for yellow/red sockets when they're +stamina or blue sockets when they're +defense or +a tank rating) or you just feel like you could use more of a rating. Otherwise there's no reason not to just put +24 stamina gems in a socket unless you will lose a meta (saying this for after 3.0.8 when our tempered titansteel gets a meta socket). Oh yeah, speaking of metas, I think EJ's theorycrafting says the best pure mitigation tank meta is Eternal Earthsiege (+21 defense rating +5% block value). The +32 stamina +2% armor value one is for bears/DKs. I know things are a little different for warrior tanks than pally tanks, because pallies value block value over pretty much anything else when it comes to ratings/values as it is a huge part of our threat in addition to mitigation. But I'd imagine it's at least worthwhile for big shield slams for you, too.
