They talked about this on Dropped Frames last night. Had a streamer there that had access for a while (JP got it a few days ago, and only played it for 25ihs hours). The streamer (Richard Hammer) there did a five day run with a friend where they took 12 hour shifts in turn (separate games). I think all he does is play xcom (long war Ironman on legendary).
Some things that came up (trying to keep them non spoilerish and comparable to official info)
- Game is now more like xcom "chess". With all the lost, the classes, chosen, etc, it is far more tactical than before. Especially considering you gain action points for things like flanking, killing from a higher position and the likes. Far more rare to do the overwatch crawl.
- You need to focus on base layout more than before.
- You need more than one "good" team due to the tired stat. You need to rotate and build your entire squad. If you do use tired people, they can get traits similarly to the "negative quirks" from Darkest Dungeon. They can apparently be really bad. Supposedly also added a "positive" that can trigger on stress/panic, but neither JP or Richard had seen that happen in their games.
- Timer for the Avatar project is there, but have more events in game that can manage it. JP said that in 16 hours in he only had done one mission for it. To offset that making the game easier, the chosen that are like minibosses that are added to your game, also have timers for them for things to happen. You need to do covert missions for those.
- Covert missions can level up stats. JP said he had sent a squaddie out to constant +hp covert missions and never used him in battle yet. So he is still low rank, but with 17 hp.
- Covert missions can end with you having to do real missions. IE: things went bad, need to go rescue that person.
- There are more "random" events in nearly every regard now. From missions, things that happen in missions, things that happen in base, level ups, research, etc. No longer do you know your "path" before even starting it.
- The "elite faction" enemies or friends both have skills that randomize differently for each game. Both your elites and the enemy elites can get sets of skills than make them OP/extremely dangerous.
- If you skip the tutorial (apparently NOT recommended for the first time since they introduce all the new things in the tutorial as well, not the same one you've done before), you can randomize which faction you start with an elite of. If you do the tutorial, you always start with the same. Richard said that skipping the tutorial makes the initial 3 missions really hard, and their outcome can snowball your entire game towards a fail or win. (He said he has restarted 20++ times, and still not beaten the game).
- The previous DLC missions are in game, but been rebalanced around the new content.
- It is very well optimized. "Fixes" the game. Loadtimes are basically not there and the game is overall more fluid. They did not talk about it, but hopefully that optimization also carries over to the PS4. Loadtimes there are crazy long.
TL;DR conclusion from them gushing over it for 45 minutes.
The expansion makes it Xcom 2.5. It makes the game better in every regard and worse at none. It is a high price for a "DLC", but would be worth it for every xcom fan. For someone who doesn't have it though, the $100 entry point (full game + expansion on Steam), is a bit steep, so might want to look to other places for the game (humble bundle had the base game for $20 not long ago) until Steam does a sale or a version with all the DLC is released.