I'd rather get stuck with a bad tank at level 30 than level 80. More importantly, I'd rather someone learn to tank at 30 than at 80. IMO, the dungeon finder is giving people the opportunity to learn to tank much, much earlier than previous, because it's easy to get a group now.

Let's face the facts: *good* tanks have always been in short supply. Hopefully the dungeon finder will give a few more people the chance to learn to become good. And if someone is completely specced/geared/buffed improperly, in my experience, I've had positive responses from offering some friendly, constructive feedback. It's not like the game really gives a lot of instruction as to how to choose your role and spec... Some people just need to be helped along--or at least told that there are sites like wow.com, wowwiki, EJ, Tankspot, etc. that can help them.

No, LDF won't 'die off quickly' - on the contrary.
I think some people are misunderstanding what the positive and negative effects of the new LFD tools are.
'Encountering lots of inexperienced players' is misunderstood as a bad sign. But instead it's positive, it's proof of 'working as intended': It is a sign of a massive increase in dungeon activity, a sign of lots of fresh player material brought into dungeons. Some of those people will be bad, yes. If they are continually bad, they will eventually stop using the LFD system, because of the continuous negative feedback they encounter there.
But a lot of previously 'inactive' players now get their feet wet and, as countless before them, realize that they really like doing this.
The most wonderful part is, that not only does this system 'allow you to keep doing same old if you prefer'. But it is finally the 'gradual path' that can bring fresh players/toons all the way up to raiding possibility (as opposed to the previous 'dividing chasm', where you had a huge gear bridge to cross, to get a toon into raiding).

Another misunderstanding is the talk of 'gearing their toons'. This system is not (just) about gearing toons. It is much more about gearing PEOPLE - giving lots of people the _practice_ they need to become competent players. (by removing the barriers to get practice).

The main and 'only' disadvantage of the LDF tool is indeed the social problem.
We have the same problem in ordinary society: In very poor countries, people are often very social, because the scarceness of physical resources means that 'human resources' are relatively of higher value (ie I don't have much money, but I know 50 other poor people, and we help each other out).
In western societies, we are very materially and mentally 'rich' (educated), but for better or worse this makes us extremely independent ('I don't need to put up with their crap, I'll handle it on my own. I can do that, I am RICH!)
So paradoxically, we become 'social misers', more reluctant to socialize and depend on others.
As has been mentioned in other posts, Blizz might alleviate this by e.g. making the LFD tool some way reward grouping with people from your own realm, and/or prioritising this
(but again, then this needs abuse protection).

I had a really great LFD run two nights ago... but I suspect you'd have to be a social player to think of it as "great". Let me explain.

Late evening, ran Forge of Souls *normal* on my T9-geared tree as I was too tired to do random heroics and it still drops emblems.

Zone in... hmm... 3 DKs - tank and two DPS - all from the same guild. Tank's touching 25k health, others well below it. Tank looks around and groans.

The first pull in any instance is an easy one to check everything's OK. Tank's health starts falling like an express elevator... OK, time to up my healing game... hmm... can't hold aggro either. This is going to be a long run.

Turns out the three of them are running random normals to gear and while the tank's learning to tank; the LFD's reckoned their gear's good enough and dropped them into FoS despite the tank not quite being def-capped (he was only a few points short, to his credit, and knew it).

To cut a long story short: none of them knew the bosses, their gear was marginal, the fights took a long time. But I stayed with them - and, to his credit, the hunter who was 3rd DPS stuck it out as well. By the end of the instance, this wavering new tank had got 3 upgrades (lucky with drops!) and had turned from "can't hold aggro" to actually a pretty good tank. And none of them had died, and they'd had *constructive* feedback and criticism about what to fix - and they'd taken it and fixed it. So much so that we then went through and did PoS with almost the same group. One near-wipe, but we managed.

Yes, this is an unusual story: people willing to help others, to learn, to spend time, even though they'll probably never see each other again. On the flip side, that's one tank on our battlegroup who's had a positive experience with randoms and might stay tanking :-).

There will be fewer 'good' tank cos of the general burn out principle involved.

Im a tank. I have had a couple of bad experiences with ninja's n asshat dps. So once i get my pet, im sticking with guild runs tyvm. Im fed up of people abusing me for their own deficiencies as im not ICC25 geared, even tho even a ICC25 geared tank cant tank WHAT HE HASNT YET HIT. So many dps must be so glued to dps meters that they cannot wait until ive even hit a mob, or even got to it yet, that im quitting the random element of DF. Healer are generally good tbh, but the stress a dps causes by 'having fun' by deciding to nuke a mob, or pull the next group = when im waiting on mana and cd's is too much. Tanks play for fun too you know, and asshats have killed that aspect for me. In our guild we have probably 20 odd tanks capable of tanking heroics (with alts etc). At least 15 of them now will respec dps to do a random, or wait for a pure guild run, rather than tank.

Cross realm LFR? No thanks, neither worth it, nor workable from my point of view. Getting saved to raid from a faily pug is something i neither need to risk, nor want to