Are your tanks dying, or feeling demoralised because they feel the system doesn't give them much personal reward?
Tanking gear's a rather hard thing to judge; there's a metric that is 'enough', and there's not a *huge* amount to gain through gearing beyond that (although, to be fair, Tank DPS isn't quite as insignificant as it used to be, and that could potentially be a clincher when you're brushing the enrage timer); plus tank deaths aren't *necessarily* the fault of tank gear, it could be healers not having enough throughput or even DPS taking unnecessary damage and in turn distracting healers.
Certainly, at the bleeding edge gearing DPS always comes first, because there's no point where the usefulness of gear to a DPSer stops at 'enough'. However, to get psychological for a moment, at the Normal level of progression, I'd be tempted to argue that that philosophy wouldn't work - when you're pushing server firsts, people have an extra layer of motivation that would make them more receptive to personal sacrifice in the short term for guild victory in the long term. At a slower progression rate, I think you run the risk of the players who don't get rewarded feeling undervalued.
FWIW, there's two other points I ought to make from a personal perspective:
In the *very* specific example of protection paladins, the two-set is great, but the four-set isn't much good unless you have the legendary helm; I don't know if similar situations exist for other tanks.
And in my role as both MT and RL, I'm generally comfortable with passing on gear anyway if there's a net benefit to others in the raid; it sets an example, but it's a decision I'm making in my own right; I frequently say "I'm needing, but I'll pass in favour of anyone else", independent of our underlying loot distribution system. That might ultimately be key, here: Ask your tanks how they feel, whether they feel like they have 'enough', whether they feel squishy (and you might want to co-ordinate that discussion with the healers), and whether they're comfortable with what they're getting.
If *nothing* else, one thing I would suggest in such a discussion is to establish if there are any specific slots they feel are badly lagging behind; while I feel like I have 'enough' in general, I do have a few 865s knocking around that would make a big difference if I could replace them, and they'd be comparatively cheap to replace (because I'd get a noticeable improvement from just a basic Heroic - or even in some cases Normal - upgrade for them, whereas other people who'd be competing with me for that slot might only really see a small improvement in the event that a War- or Titan-forged item drops.
