Traditionally, Pokemon games revolved around random encounters with wild Pokemon, where you caught your little monsters by weakening them before chucking as many Pokeballs as necessary. Let’s Go removes the randomness entirely, so now you can see every wild Pokemon before you encounter it. You also don’t battle the Pokemon you choose to encounter, instead only catching them with a system inspired by the mobile hit Pokemon Go. And honestly? I kinda love it.
No random encounters really changes the game feel for me, I can't express that enough. In past generations, caves were stressful because you're constantly hounded by high encounter rate Pokemon like Zubat, and maybe your party gets slowly beaten down and look! A surprise rival fight! Hope you have enough potions buddy! Let’s Go removes all of that by letting you see all those Zubat, so you can avoid them all you want. Already caught every Pokemon in Mt. Moon? No problem, you can evade all the others you see.
Since you aren't battling the wild Pokemon you DO find, you're never at risk of your party getting knocked out just so you can catch something rare. You also gain experience with every Pokemon you catch, so you’re still able to level at a fast enough rate to take on trainers and gyms. This essentially removes the need to grind, which was the other main reason wild battles were necessary. It’s turned the entire game world into a version of the original’s Safari Zone, where the Pokemon don’t fight back and you’re only limited by your access to Pokeballs.
I 100% get people saying that removing wild battles also removes the challenge, but consider for a moment that maybe random encounters are artificial difficulty. They’re a way to slow the player’s progress without actually threatening them too much. If you’re losing to wild Pokemon, you’re doing something wrong. Zubat don’t make Rock Tunnel dangerous, they make it arduous and annoying.
Losing wild battles gives you the freedom to just BE in the world without worrying about getting stopped by a meaningless fight. It’s made me realize I don't think I ever really wanted to FIGHT wild Pokemon, it was just a requirement in order to catch new ones and gain levels. In Pokemon Sun, I stopped catching Pokemon I wasn’t going to use; looking for random fights, running from those I didn’t want, then trying again to get what I wanted became such a chore. That’s changed in Let’s Go because it's entirely up to the player who they want to catch and when. They’re no artificial penalty for trying to catch my 3rd Rhyhorn, I might as well.
The system isn’t perfect. I personally would like something with a bit more finesse than the ring system borrowed from Pokemon Go, and the forced motion controls have some already well documented issues. The basic concept is much better than I expected though, and when the promised “core” Pokemon game comes next year, I would be totally fine with random battles staying out.