Samurai Job = DPS Job

[Mimilu]

FFXI: Ninja (DPS?) - FFXIV: Ninja (DPS)
FFXI: Dark Knight (DPS) - FFXIV: Dark Knight (tank)
FFXI: Samurai (tank?) - FFXIV: Samurai (????)

Samurai is a DPS in XI, they did originally design it at a tank that focused on parrying but they left that on the cutting room floor and had it focus on Weapon Skills (skills that came from the weapon type the weapon you were using).
Ninja is a DPS as well but they can also be used an Avoidance Tank, using evasion and shields to avoid damage. Ninja is also one of the best jobs to use as a sub-job.

[Exstal]

Samurai is a DPS in XI, they did originally design it at a tank that focused on parrying but they left that on the cutting room floor and had it focus on Weapon Skills (skills that came from the weapon type the weapon you were using).
Ninja is a DPS as well but they can also be used an Avoidance Tank, using evasion and shields to avoid damage. Ninja is also one of the best jobs to use as a sub-job.

I was more alluding to the fact that classes that were used one way became the opposing role;

Ninja, somehow as a tank => DPS in FFXIV
DRK, dps => tank in FFXIV
SAM, supposed to be tank => (let's hope Naoki follows through) DPS in FFXIV