For those worried about exclusives, that's where making a Pokémon Go account will probably come in handy. Pokémon go doesn't have version exclusives, and since you can Port the Pokémon over to Let's Go and Pokémon go is free to play, it makes it doable without having to buy a whole new system. Plus, if you get the pokeball accessory thing, it can catch Pokémon while you're walking like in Go, so you don't have to keep your phone out all day looking for an ekans. You can also evolve Pokémon without trading
Building on what PirateKing said, I do think there will be a few surprises for us. Now, I don't expect them to be like, the battle frontier or anything, but I think it will be more side-like features that will be surprisingly fun, since this game is more casualized (doesn't look like it has EV's, etc.) I think the Co-Op mode is going to be the real big thing, and the following/riding Pokemon is going to be sentimental for veterans. I mean, I remember back in the days as kids...
"Oh, is that Pokémon, can I play?"
"Well, not really, I mean it's one player..."
"Can I try yours?"
"Well, these are my Pokémon and I kinda raised them, I don't want someone else leveling them, and then like, I can't battle these gyms again and was looking forward to it... Why don't you just watch over my shoulder or get your own?"
MEANWHILE, LET'S GO-
"Is that Pokémon, can I play?"
*Throws second joy con*
"Sure, jump in."
It's going to be a really great feature I think, especially for people to play Pokemon with casuals. My husband is not a Pokémon fan and we don't play a lot of games together cause we have totally different styles. But this, this we could play together. A leak released on April 1st that's been pretty accurate so far said there's an online hub, and I could see some neat online features being integrated for battling and trading. I hope, since they have all 152 Pokémon animated with riding features for the big ones, that we might even see some mini games like pokeathlon in HGSS. Wouldn't it be neat if there was like, a racing game?