Yeah, it's a hard question. There are a lot of things that are related to good design, but aren't mechanical/something that can be programmed directly, per se. There's also a lot of subjective feeling, min-maxing stuff, hot big a gap between playing well and playing perfectly, etc that will involve things like numbers tuning and how one feels playing it, but may not EXACTLY be design. So it's kind of a hard question, just one I've been thinking a lot about lately.
"all Jobs must have a Job Gauge" + 2 min meta + proliferation of builder/spender systems = Jobs feeling samey
I'm not even sure that design is BAD, it's just bad when every Job pretty much uses it. It also makes every Job a "burst" Job. Some other MMOs have things like "sustained damage" and "cleave/AOE" strengths, but we just have all the same across the board and every Job is a burst Job. Even BLM, praised for not submitting whole to the 2 min meta, has bursty potential and does so with raid buffs in optimal play.
I think the Jobs I like best in terms of design right now are probably PLD and SMN. Yes, I know, hot take, but the REASON is what's important.
I like that the rotation is kind of broken up into modules that you can move around. For example, say you need to raise someone as SMN. You can without totally borking your rotation. You have one (and some change) Ruin 3 casts per minute that you can substitute. Say both healers go down and you have to raise both. That's about 10 seconds of time. But you could do this and then before your next Demi, you just use a Primal, maybe only get off one of its attacks, then roll into the Demi, cutting it short but keeping your rotation in line with raid buffs. The key here is that the rotation is flexible and allows this.
PLD is similar with Holy Spirit and Atonement, and even non-Holy Might Holy Spirit casts that you may be inclined to use in some situations (extended disengages, for example).
Note this isn't an argument of difficulty.
The key here to me is, mechanically, the Jobs allow choice and allow you to tweak and bend your rotation on the fly in response to situations, to push or pull in response to party needs or boss mechanics.
Contrast with very rigid Jobs, like say GNB or DRG, which are much more punished if they drift or fail to maintain their rotation. There are also Jobs in the middle, like WAR, where it needs to upkeep its Storm's Eye buff, but otherwise has a lot of flexibility in when it can use things other than Inner Release clockwork.
Other than maybe BLM (and some healers, maybe?), we don't really have any sustain Jobs anymore, and even BLM and WHM get in on the 2 min burst business. And it's not bad to have rigid Jobs...as long as they aren't ALL that. But for me personally, I think good Job design allows for flexibility and choice on the player's part. I think that's also where real skill expression comes from. Not from having one "right" answer and a bunch of "wrong" answers, but from having several "right" answers and the player is rewarded for thinking through how to use them, what else the encounter does, planning around that, and reacting to party needs.
I DO think it's good and healthy for a game to have a lot of different types, though, since different players like different types. There have been points where I enjoyed really rigid classes since I could kind of "learn/get it" then do it consistently. Some people love proc based ones because it means they never have the same encounter twice. For me, I think the kinds I like best are those that have some "anchor" points, but flexibility in between them. SMN's anchors being Demis, PLD's being FoF/Requiescat, but what you do in between is much more a matter of player choice and skill expression.