I feel like it comes down to what functions should be deliberate/separable among each, and what that's supposed to afford.
For PLD, support feels like it should be a key, core, deliberate feature. Intended affordance: On-demand (esp., emergency) support. Thus, Clemency (especially at its 20% more powerful and Req-affected potency) made a lot of sense. Cover, especially with neither a gauge cost nor built-in additional mitigation, made a lot of sense.
For WAR, there's little that feels like it ought to be separable, while the intended affordance appeared to be the ability to be less affected by functioning outside of normal party support (having a bit more burst damage potential in itself to shut down key targets, its sustain being less context-dependent as to be less dependent upon timely help, etc.). Thus, self-healing being tied to and around damage-dealing made sense.
For DRK, offense and defense were to be a bit more separate of choices than WAR, but from a shared resource, such that it offered, as its affordance, timely control of that balance. One was to be rewarded for correctly shifting inputs towards their variably useful tools to greatest possible output. They were a bastard with a bag of tricks, with a higher than average performance gap from those tricks' utilization.
And I've no idea what GNB was ever supposed to have meshed or afforded, as it's never felt like anything more than a fairly rigid DPS with some tank CDs slapped atop it.
I'd jump for the soonest opportunity to play GNB if that were what it actually was.Gunbreaker: A fleet-footed bodyguard who evades and outmaneuvers foes, tactically employing the recoil of his own attacks to retreat or engage.