Paladin:
Offensively, a constant rotation between combos. Upkeep a combo'd dot, use RA to gain Sword Oaths, Burn them on atonements within burst windows or drop them for alignment. Switch to a spell combo once per minute to expend MP and regain HP, all while semi-frequently using oGCD attacks.
Defensively, Has high %mitigation and a consistent trickle of passive damage reduction.
Warrior:
Offensively, Build up gauge by using Storm's Path to expend on Fell Cleaves while maintaining your Storm's Eye buff. Has few oGCDs and power is largely expressed through big heavy hits with guaranteed direct crits.
Defensively, has lower mitigation and instead has numerous ways of regaining their own HP, from a direct self heal, to a pseudo-lifesteal, to increasing your maximum HP.
Dark Knight:
Offensively, Use your filler combo to build up Blood to expend on Bloodspillers, while making sure you pool enough to use Living Shadow on cooldown. Very high frequency of oGCD attacks, constantly burning MP to keep up Darkside and deal extra damage while pooling enough for TBN.
Defensively, entirely nullifies certain amounts of damage and clever usage of the ability effectively refunds the MP cost. Very effective against magic damage.
Gunbreaker:
Offensively, a very busy rotation with several cooldowns, both GCD and oGCD, that need to be kept rolling. Fairly slow and static outside of burst windows but with a plethora of different attacks that need to be crammed into No Mercy. Cartridges effectively act as a fourth step to your combo but with much more versatility, allowing them to fit into the job's frantic one-minute burst.
Defensively, has something of a "bag of tricks" - Conditional healing, gradual regeneration, moderate %mitigation, Parry rate increases, even expending their own HP for invulnerability in the form of Superbolide.
The only real point of similarity here is DRK/WAR, and this was a point of contention for a while anyway. Even then they have a fair few differences, what with DRK's faster, oGCD-oriented gameplay, and WAR's slower, heavier feel.
Now for healers:
Press a DoT every 30 seconds while mashing a filler spell.
You get 1 extra button depending on your job: A rebate for wasting GCDs on healing (WHM), An extra GCD to throw in there once every 45 seconds (SGE), A gauge dump to make you stop healing people(?!) (SCH), or a card twice a minute to pretend that you're totally a value to the party that couldn't be harmlessly replaced by giving everyone else two more goddamn buttons.
Try not to break your 1 key from mashing it too hard.
If an entire role could be seamlessly removed by giving everyone just two more buttons, without redesigning ANY of the game's content, it is redundant and should either be removed or completely rebuilt.