I'm not sure however that the changes you propose are ideal though. Just to throw this in, personally I would rather see normal&piercing opposed to elemental&shadow. The first are the "normal" damage types, the latter are rather "magical" in my opinion.

Think of a tank, they have a lot of Normal defense. All that thick armour and firepower protecting them from impacts, bullets and other deadly things.
Now, imagine an RPG. An RPG is piercing damage, they actually concentrate high explosive ordinance right down to a needle point to punch straight through that armour and blow a hole in it.

There's no defense against that for most tanks, without additional protection or special active systems that cause the missiles to detonate before they hit the tank.
Which aren't commonplace or viable in most combat situations for armoured vehicles.

Obviously, viscous fluids would be the thing to absorb piercing attacks. High calibre armour piercing rounds actually disintegrating on contact with water.
But dropping a brick on that water would break it's surface tension in a snap.

Now do you understand why Piercing and Normal are opposites?
Because in real-life, they are opposites.

Don't have a rationale for Elemental or Shadow, but if you really need one I'll make something up.

As said before, the problem is that you're faced with too many types of damage during a run through a gate.

The idea is you're meant to choose the lesser of two evils. Becoming an unstoppable he-man shouldn't be an option, ever.

Yes, they are quite silly compared to vitapods. The only real difference you can notice with it is during PvP, but there you can only have 1, 2 or perhaps 3 extra bars. I like your proposed idea for armor offering 3 styles of defense. Perhaps though it would be even better to not have it armor-item bound, but rather as a "vital part" of your armor item that you can swap? That might need a larger rework of armor though.

The main idea was appeal to players of three distinct styles:

Cautious: Health
Calculative: Defence
Capable: Ability

Players who don't know much about the game might opt for more health to get a blanket defence against all damage, but low resistances to it.
Players who know what to expect and tend to be more efficient in their killing may opt for high defence to reduce the risk of fighting in dungeons.
Players who have lots of skill or just enjoy being on the knife's edge can choose ability armour, to enhance their skills and chance of survival.

I don't really understand the last part.

Perhaps Vitapods should still be kept as an in-game drop though, but then a lot smaller? And as mentioned before, some vitapod-alternatives would be nice aswell.

Vitapods are unnecessary, player health shouldn't increase as you go deeper, it should increase as your gear gets better.
If depth scaling is so severe you need to offer players more health at every depth, that's a balance issue.

Like I said: Since they've been beefed up, it's made the balance patches pointless, because all you did was double monster damage, then player health and then player armour. The only thing you need to double is weapon damage and we're back to how the game was before.

You can only loose half a bar of health at the least.

You can lose less than half a bar, it just doesn't have a visual correspondence.
There was an issue with the revive window appearing when your health was below the last half but not yet zero that resulted from this.

I was thinking along the lines of armor having a core element that would provide either 3 bonuses you suggested. That way you could still have the option for which damage type you wish to "specialise" against, and be able to freely choose your armor-play style.

One of those playstyles is having extra defence.
So, no, I would find that redundant.

I thought more in the direction of other MMORPG's

You mean MMORPGs that only have melee damage and magic damage, as opposed to four very specific types?

Just encountered something pretty stupid with armour Variants.
I got a Piercing Resist: Ultra on my Seraphic Helm and it only adds 25 points.
That is incredibly useless, to say the least.

If you're going to offer unique variants that don't add to pre-existing stats, at least make them worth the time to acquire.
An ultra bonus should be at 75% of the highest armour value on the item, not some dimensionless number.

Something needs to be done about this, eventually.