I saw that Elunedra had brought up the Dodge/stacking Mastery on hit tanking trinket. After messing around with it in 5mans, I'd say the trinket does have a use... just not likely in current content. If you find yourself in an encounter where you're basically non-stop AoE tanking a decently large amount of adds where their primary source of damage is melee, the trinket could get you to 75% DR from just armor alone for extended periods of time. Unfortunately, if you're single-target tanking, the trinket does not proc enough to matter, let alone get multiple stacks (since it's a 4% chance to proc on dodge). If the mastery proc had the proc rate like Talisman of Bloodlust (the agility/stacking haste on hit trinket), the trinket would definitely be much more useful as a pure melee mitigation trinket.
And as most people have been saying, once you go expertise hard-cap as Guardian, you cannot go back. The reliability from all your rage-generating abilities hitting when you expect is a lifesaver as a rage tank (which is why prot warriors tend to do the same thing), not to mention it's pretty nice DPS.
With respect to the legendary metas, I'd definitely have both sitting on stand-by as a Guardian druid. Unfortunately for me, our raid is cursed with respect to Vanq tokens (while some Conq/Prot token people already have 4pc off-spec, some Vanq users still don't have 4pc for primary spec), so I haven't been able to get a second helm using bonus rolls on Twin Consorts and Jin'rokh... but the second I do get a spare helm that can use the DPS meta, I'll still use it even on some 25man heroic fights (gets a little expensive if you feel like switching the meta non-stop for certain fights in the same helm). The main purpose of having the DPS meta is to push more DPS as a tank... sounds a little redundant to say, but with proper execution you can do massive damage even just using the tanking meta gem. Disclaimer: use of the DPS meta likely depends on your healers, since the proc on the tanking meta is pretty godly.