What the title says. Ignore balance/tuning numbers for a minute and just consider gameplay. What the rotation feels like, if it covers all the bases for what you enjoy in a Job, etc. What things do you like about it? What does it have/do that makes you feel good playing it and feel like it's a good Job?
I don't want to steer the discussion too heavily, but by way of example, I can go first:
RDM:
I really think RDM is a very well designed Job. The basic concept is so easy pretty much everyone can understand what you're trying to do within minutes of picking up the Job, yet there are a lot of little optimizations you can do to improve, and the Job rewards high level fight knowledge just like BLM does in terms of when to use your burst combo, if you need to save Acceleration for things, and whether even really high end play of planning out your Mana gauge across a fight.
Meanwhile, it has easily understandable utility. Verraise is very clear about what it is, Vercure can be useful in some situations (but also for skill ceiling used for optimization Dualcast procing when bosses are untargetable but returning soon). Embolden is straightforward and useful, while Magicked Barrier (along with Addle) is useful for additional planning or emergency use situations. Vercure and Verraise are powerful, but at the cost of disrupting your damage rotation (so you cannot be healer AND DPSer at the same time, a good trade), preventing RDM from displacing healers while also preserving its clutch utility in emergency situations like a healer DC in a 4 man.
It's as easy or hard as the player wants to make it, entry level and high end, and with lots of utility that is well balanced and understandable, and pulling off the burst combo just feels very satisfying, as does Slide-into-Dualcasting your way across the field, dancing through mechanics. Indeed, the rotation itself largely feels buttery smooth to play, even if I personally somewhat dislike the proc system, it still feels good overall in general to move between the short casts and Dualcasts, building up to the burst combo, zooming in to do it, flipping out, and finishing with a bang before starting the process again. It feels responsive and smooth all around, and just feels GOOD to play.
And when it comes to leveling/syncing, the Job fills out early, establishing the core concept and rotation of the Job by level 50, and a more or less complete rotation (Embolden and Manafication) by 60, meaning other than not having Verraise, you get NEARLY the full RDM experience even when synced to mid-level (50/60) content, where most of the game (roulettes etc) are likely to place you on any given day. (Not as good as, say, WAR, but better than, say, BLM, which has to change its hotbars and rotation every few levels).
I feel it's probably the single best designed Job in the game. Not PERFECT (I hate the AOE rotation is "backwards" from the single target rotation, I think Enchanted Reprise should cost 1/1 or 2/2 instead of 5/5 MP or should cost 15/15 but 3x of them can take the place of your melee [for when it's not safe to melee] to unlock your burst but at the cost of a DPS loss traded for that safety, and I think that some spells should come at different levels, like Vercure should be by level 15 and Verraise by level 50), but very well designed overall.