So some things I noticed from the Playhouse playthrough of "Let's Go":
Poke Ball Plus & Mew: I guess this makes sense. Stinks they won't have a free event, though I guess you can also transfer a Mew from GO so maybe they count that as the free event. Though unless they make getting Mew mandatory for completing the Dex I don't think it's such a big loss. These games are spin-offs and even if there's a competition scene around these games it'll probably be small. If you don't get a Mew for Let's Go it's not that bad, so having it as a bonus for getting the Poke Ball Plus I think is alright (and hey, since they're having that dual game and Poke Ball Plus pack maybe it'll give some people the excuse to buy them).
Party Gains Experience With Captures: Well that answers one major questions. Outside of Trainer Battles how was your Pokemon suppose to gain experience? Every capture gives all Pokemon in your party experience. Now does this make them removing wild battles alright? Of course not! If you want to bridge the two playerbases together I think letting you weaken the wild Pokemon (and in return letting it attack your Pokemon putting them at risk, thus a reason for you to question whether to continue doing the battle or running away) while also having the GO's way of throwing the Poke Ball would still have worked better. However after watching the Treehouse I'm having another feeling why they changed it to be like that... and I don't like it (looks down to the rival change to hear my theory).
Wild Aura: I noticed after catching a Pokemon, other Pokemon of that species would sometimes have an aura around them. I wonder what that is, an indication that Pokemon is slightly stronger? Can you find stronger Pokemon the more you catch a member of its species? That would be one reason to capture more of the same species, at least until you reached the limit.
Different Heights & Weights: I was right! Each member of the species have slightly different heights and weights, the Dex recording the biggest and smallest specimens you had caught. And it looks like when you encounter an especially small, big, light, heavy Pokemon you'll get a notice of the Pokemon having that unique trait. Though, why? Is there going to be a mechanic or competition which involves this?
More Pikachu & Eevee: So you can get other Pikachu and Eevee in both versions. Well, at least Pikachu. Curious if Eevee is going to be put someplace in the wild. Wouldn't seem fair for Pikachu to remain an easily obtainable wild Pokemon while Eevee is kept as a gift Pokemon.
Special Partner: Speaking of which, as talked about above, looks like the stats of the partner Pikachu is higher than usual. This is great, unlike in Yellow it means the partner Pikachu and Eevee won't be dead weight. Not only that, but they looked to have changed/added to their movepools. Pikachu getting Double Kick (and wouldn't be surprised if Eevee got it too) would help with battling Brock using them (who's Pokemon is immune/resistant to their STAB). Reminds me of them giving Charmander Metal Claw for this exact reason in FRLG. Curious what other moves they might have given them.
Your Rival Trace: ... NO! BAD GAMEFREAK!
Looks, I'm fine with them changing somethings around in sake of trying to "bridge" the fanbase or just to have something different. But changing the Gen I rival? GameFreak, are you stupid?
Oh, but I know why they did it... which makes this decision even more stupid and infuriating. They changed the rival from Blue to this Trace because they didn't want to hurt the little kiddies feelings by having a mean rival. Trace gives you helpful info and free items while Blue would insult you and provide a tough battle. GF were afraid kids were too much "special snowflakes" to handle a rival who actually, you know, opposed them.
This is by far the BIGGEST mistake GF made. Why? Because it completely removes one of the driving force of the game, or at least lessens it. Why did you want to become the Champion? Because you rival is a jerk who thinks they're superior to you and only way to prove them wrong is to become the strongest trainer in the region by becoming Champion. When you became rival in the Gen I games by defeating you rival, it's a very cathartic and satisfying feeling, not only have you proven you're the best but also knocked them down a peg (which later causes him to go through character development we see in later games like HGSS and Gen VII). But now that the rival is friendly? Even if they change the scenario to being a "race you to the top" it removes any urgency to become Champion or the tension you feel when you're having the Champion battle. It won't feel you're overcoming a great obstacle, you're just having a battle with a friend. No matter how tough it is, there won't be any emotional intensity to it.
And I'm not slamming friendly rivals, but note that until USUM we haven't had one as Champion since Gen I. It's not just because they already did that and so they can great an interesting character as Champion, but friendly rivals provide a different push for the player. They're encouraging and want to see the player succeed, so having them as Champion makes it feel like they're making themselves a stepping stone than an obstacle. USUM is an exception because YOU were the Champion so you were the obstacle that Hau was trying to overcome, not the other way around.
I'll be honest, this might have made me lose the most interest in getting the game if I were in position to get it. Yes, it may sound shallow, but to me this is the crossing line into unacceptable changes. This isn't a change to draw in players or try something new, it's a change made because GF underestimates the little kids who will play these games... even though when Pokemon first came out the kids back then had no problem having a jerk rival you want to beat (I being one of those kids).
Slowpoke Side Quest: So looks like they added those USUM-level side quests to Let's Go. Which, alright, though a bit annoying you got a Big Pearl for doing nothing. Like, at least have the player have to do a battle or something. And, asking the same question that I asked USUM, how much of these side quests are there that they could have instead used to develop the important NPCs?
Gym Quiz: Okay, the level of handholding is getting ridiculous. REALLY? We can't enter the Gym unless we have a Grass- or Water-type? And what if I have a Fighting-type? Or what if I have a Pokemon which has a move of those Types? Also having the weakness isn't necessary to beat the Gym, you can have a neutral battle and still win and the awards aren't any different. Come on GF! Kids figured this out in 1998 without this!
Good Ol' Brock: Well at least they didn't change the Gym Leaders... or at least the first one (I swear if they changed Giovanni from being the last Gym Leader). Brock's looking good, and I like the change done to the Gym with having an audience area (though does that mean people just sit there waiting for trainers to challenge Brock or does Pewter Gym have a secondary function?).
BTW, can you do the two player co-op against the Gym Leader? That kind of feels like it cheapens the experience. Who needs type advantage when it's a two on one?
Poke (Park)ing Out The Safari Zone: So they replace the Safari Zone with the Poke Park, which I guess makes sense as the entire game now works like the Safari Zone. Guess the Pokemon exclusive to Safari Zone will be distributed to other routes.
Candy Here, Candy There: Ah, so a Pokemon sent from GO leaves candy behind when transferred to Let's Go, I suppose that's a good reason to transfer them there instead of to Willow (though there has to be a limit or else there would be no reason to ever send a Pokemon to Willow again).
And in Let's Go you can send extra Pokemon to Oak who'll grind them exchange them for candy that increases one of your Pokemon's stats. I guess this is how they're getting rid of IVs and EVs, in order to really get your Pokemon stronger you'd need to catch a lot of Pokemon and send them to Oak for stat raising steroids candy. And if I'm right that catching more Pokemon lets you find stronger Pokemon of that species means once you get enough candies to level up the one you grinded you'd then want to catch another one which would become stronger if you trained it up instead. Thus, sort of cheapening the feeling of raising up a Pokemon you caught when it might be better to catch and train up another one later after having caught a batch of them.