The introduction of mastery in Cataclysm is having a pretty significant impact on how you, as a tank, will view gear. Simply put, mastery is something that every spec can put to good use because it's designed to be universally applicable. What this means is that the playerbase has a significantly larger pool of items from which to draw and that the pieces themselves need to be viewed on a stat-by-stat basis. Of course, this applies to pretty much every class but perhaps more to tanks than any other role due to the removal of defence rating. Sorry, but it's example time.
A pair of plate boots just dropped from a raid boss. They look like this:
Zellviren's Boots of Bluster
+100 Hit Rating
Now, depending on what is being worn around the raid, these boots could well be attractive to both tanks and damage dealers. For me, I'm looking at hit rating and Critical Block; in my offspec, I'm looking at hit rating and Strikes of Opportunity. Both stats are things I want in either role, which equally means that Roarzol, my raid's Arms warrior, could be equally interested in the boots. Instead of merely looking at the iLevel and deciding that "they iz tank bootz" or they izn't, the playerbase is being forced into looking at what a piece of gear could potentially be giving them. Mastery is a tanking stat. It's also a DPS stat. In fact, it could easily be argued that hit rating, expertise rating and mastery (in any combination) could be considered viable tanking options in lieu of something more obvious.
Naturally, this point may be counter-weighted by an assertion that parry and dodge now become the new "defence rating"; if a piece of gear has one of those two stats it's a tank drop and, if it doesn't, it's a DPS drop. However, that line of argument only holds weight if it can be reasonably proven that hit rating, expertise or mastery are clearly undesirable for tanks - and no such proof will ever exist. This is especially true when we consider the other big change to gearing.
The ability to reforge gear cannot be understated. Basically, the ability to remove half of a less desired stat in order to gain an appropriate value of a stat you want more is huge. The argument that tanks should stick to gear with parry and dodge is blown to bits with one trip to the reforger. All of a sudden, our boots look like this:
Zellviren's Boots of Bluster
+50 Hit Rating
Voila. Bona fide tanking stats.
Mastery really is all things to all men, while potentially opening up gearing avenues that were previously unheard of. The ability to reforge off slightly undesirable stats for something you like better just adds to the level of customisation. Parry and dodge on gear will stop your own items going up in smoke, while you can also start laying claim to things that are potentially decent upgrades despite not being perfectly itemized for you.
The implications here are (I'd say) threefold.
1) An upgrade is an upgrade is an upgrade is an upgrade.
We're back to where we were in vanilla. BiS lists won't go away, but players should start to look at upgrades with a far more open mind. The developers are using mastery and reforging as a means toward freedom in creating gear that doesn't need to be perfectly itemized. If a piece isn't perfect, you can look at the potential in it and then make an assessment of how you can juggle your other slots to make it fit in nicely. Those who like mindless and unambiguous upgrades are going to feel the pinch here, while those who enjoy gaming their gear to get the best out of it are going to have great fun.
2) Tanks are going to be spoiled for choice.
As hinted above, nobody other than a plate tank can lay claim to pieces with dodge or parry on them; they're yours and they're safe. However, anything else that has a mixture of hit, expertise or mastery is fair game if it's going to be good for you. A spec that has traditionally seen a very tight line on gearing up due to defence rating has now been freed to grab pieces that may not be perfect, but could well be very good while you wait on the ultimate upgrade. I'd even argue that haste isn't terribad if you reforge most of it off, but you're pushing it if a plate DPS also wants that drop.
3) The potential for drama has just shot up in PuG raids.
It's obvious that your loot system, whatever it is, should deal with this particular issue regarding guild raids. However, if you mainly attend the higher PvE content in a PuG, I would be ready for arguments. With tanks laying claim to a larger share of the loot table, and with said tanks having little to no interest in the rest of their raid, it's not a jump to suggest that they'll happily roll on drops that will agitate plate DPS players. We've already seen it in Icecrown Citadel when certain casters have argued over cloth that did or didn't have hit rating on it and who, as a result, was entitled to roll. Ouchy-ouchity-ouch.
I've already said that Cataclysm is changing how the players view the game in hugely significant ways and this is yet another that tanks, particularly, should be considering. As you're levelling, you'll be shocked at how often you say "hey, that quest reward could be okay". When you hit raids and you're juggling stats, that notion changes to "actually, I could fit that in nicely". Ultimately, the gearing spectrum just got a lot more colourful for tanks, and in a way that allows for a previously unthinkable level of customisation for the imaginative.
Ghostcrawler argued early on in development that removing stats didn't necessarily remove depth and that we'd have more choice than ever before.
Originally we scoffed, but it's just as we were promised; we're shaping up nicely.