The real win for GS was making the R&F content good. Governors and Loyalty are very solid systems now. T3 governments are better. They even made Emergencies decent!
The civs added are really good--unusually well made. Balance is all over the place, but the content itself is all home runs. (Even trashy Canada's design is good, if you squint really hard.)
I was pessimistic about random storms/volcanos, but it's... actually quite good!
On the flipside I was hopeful about Power, and it's a mess. T3 buildings were already a noob-trap not worth building, and now they require expensive infrastructure for the privledge of doing so? Uh, no thanks, I'm good.
I was also pessimistic about World Congress; it's better than the junk in 5 and exceeded my expectations, but is otherwise weak? Favor works great, yet many of the votes are goofy, and the victory condition is a mess. It slows the game down even more with even more clicks, and that's before bring all the new AI trade requests...
The Climate Change whatever is... unfulfilled? It's not so much a disappointment as missing, sans for a few global effects that suddenly show up without warning in the mid-game 2 turns after a coal plant appears. There's no political interaction, no tough choices, no long-term environmental consequences. It's just, not actually a part of the game.
Grievances are a key improvement to the most sloppy and opaque aspect of Civ6 AI. Breath of fresh air, and has great rammifications for how the AI handles Religion.
Railroads, Tunnels, and Canals are cute.
Science Victory changes are a welcome two-steps-forward-one-step-back. On one hand, the 50-lightyear buffer provides a much needed backstop to prevent runaway players following exponential growth straight into victory. But now Science Victory's dramatic conclusion is just waiting for a bar to fill, with no counterplay beyond "win first?"
Cultural Victory is similarly improved with a caveat. The game badly needed something like Rock Bands, a "finishing move" for CV. But it's a little underbaked, very RNG-dependent, and very overpowered.
The updated strategic resources are the most underrated change--the previous system was asinine and did not represent the geopolitical dynamics we'd want to see in gameplay in the slightest. Now an oil-hungry empire needs more oil! But like the other brand-new systems, the math is off, and resources other than Uranium (and rarely Oil) are way too plentiful.
At the end of the day, I have no idea what scale I'd rate a Civ expansion on, much less what it would be on that scale. (What am I comparing it to? The other 2nd expansions to Civ 6?) But I find myself wanting a third expansion, if it will polish up the GS content as well as GS polished the R&F stuff--that polish was the real value here.
What concerns me is that both Civ6 expansions totally missed their titular themes. Past Civ expansions had broad-and-grandiose titles, and with Civ 6 the team tried to have a specific theme or narrative at the heart of the marketing. "Rise & Fall, Golden Ages & Dark Ages!" "Gathering Storm, Climate Change!" But these were kinda gimmicky duds, while the real value was in the polish and content. I just want to pay for more of that (accompanying gimmicks or not, either is fine), but the team clearly feels the need--to survive in this market--to establish a more specific narrative for the expansion product. And if they aren't delivering on that perceived need, that imperils the business case for any mythical third expansion.