It would be cool to see this feature return!

[Magis]

they mean it would involve completely redesigning every encounter and re-devising every rotation for every fight.
We have close to 90 different instanced encounters with probably another 30 in development. Bringing it back would basically be saying "nuts to that!" and rebalancing every single fight to accomodate a feature they didn't feel was worth keeping probably for this exact reason.
If a boss needs to have a body part destroyed, it's easier to just make that part a separate entity like with the five-headed dragon, the qarn HM boss, the toto-rak final boss, etc.

No it would not? Pre-patch/expansion fights would be the same as always while later ones would incorporate weakpoints. We already have "targeted limbs" (boss in Sunken Temp, WoD Hydra, boss in Toto-Rak) on some monsters without breaking the game or having every mob redesigned, how would it be any different with just having a flag on new monsters vs old ones?

Also, I wouldn't be surprised if some of that original underlying code is still there. We still have elemental and weapon type specific damage (blunt, slashing, etc), they are just "disabled" currently. Check the combat logs, and it will say "monster hit with <element> damage".

Doesn't really matter if it is boring or not. The system is just not in place, look at Monk for example. Cannot use X skill unless in X stance which means they are forced to do a rotation anyways. The only possible change that would be "easy" to implement for them would be to change it so you break a limb by doing X skill form Front instead of Rear and things like that. However it really isn't necessary, we already achieve this by allowing things like "Titan's Heart" become target-able. This is just adding fluff to the battle system that could use other improvements currently (mudra lag fixes, removal of pointless abilities, changes to the cross class system).

Well, seeing as SE doesn't seem to be afraid of changing base rules (ie: no base class for exp classes), I don't see this being a problem if they were to go ahead with something like this. Also, if a player was going to fight a certain monster with a certain weakness, would they not build their strategy/rotation around that fight? Wow, going in with the same rotation you use on every monster may not work and you may have to change things to cater specific monsters. Everything is fluff, you can break the game down to "make the number go higher". It makes the game less monotonous and goes beyond the memorized "do job's rotation" and "do the certain mob's dance".