I was severely disappointed to see that it was being removed, and every time I play the game I look at this mockery that it was replaced with in dismay... yet the irony is that I cannot shake that off.
Edited: Apparently the character limit wouldn't let me post the original message, and my copy/paste failed so now I don't have the post anymore.
The new Shake it Off (8-24% hp shield, uses up all your defensive cooldowns for maximum effectiveness) is something that really only would be used in two situations: when everything is going well and thus you don't really get much benefit out of it, or when everything has gone to shit and it isn't really going to save anyone. Because it ends all of a warrior's defensive cooldowns that means that it should not ever really be used as a main tank; only as an off-tank and only ever when there is no chance of needing to switch in as the main tank for the next minute or two (meaning not savage content, or most things that have mechanics that require tanks to switch).
In truth, the best possible situation that I can think of for it to be used would be just before a party-wide damage mechanic that is also a tank swap, using the ability while the buffs it consumes are only a few seconds away from expiring, and also within the timeframe (15s) of keeping the shield active when the party-wide damage goes off. However, as most content of this sort is such that a single AoE shield from a Scholar or Astrologean would be able to cover the party 80-150% as well as Shake it Off, it hardly seems like a necessity and instead would simply be something that is there to save a single cast and some mana on a healer. So obviously the maximum usefulness would be in a party that has 2 white mages, because it would be the only party-wide shield.
If the warrior is off-tanking and uses this ability, their defensive abilities would be on cooldown if used to full effect, and if used at base value 8% of maximum hp is not a shield that will do much to mitigate damage to most of the party. It would also prevent the warrior from swapping in for the main tank if needed because nearly all of his/her defensive cooldowns would be unavailable. For 4-man content the damage mitigation from defensive cooldowns is going to be more useful to the party than the AoE shield as most mechanics in 4-man content are not the sort that would be able to wipe out a party without shielding.
So again, I really do not see the usefulness of this ability outside of trying in vain to save a run that has more problems than 8-24% of their HP... or being icing on the cake of something that is already going well.
Old Shake it Off had a number of uses, and there are a lot of things that you could do with the old version that the new version cannot.
1. Removing all (not one) removable debuffs can save several casts of Esuna in some fights, freeing up a healer to focus on other things.
2. Being able to remove an effect like Heavy or Vulnerability can mitigate more damage to the tank than the shield would be able to during a fight.
3. Fixing hilarious DF mistakes, like healers that don't put Esuna on their bar or don't know to/don't pay attention to removing Doom in v4 or Dun Scaith.
4. Soloing SB A ranks. You can't do it nearly as well as with the old Shake it Off.
In summary: I miss this ability, because it let me do things that I can't do now and what they replaced it with is useless in nearly every situation except when everything is already going exactly according to plan and therefore you don't need it.