tl;dr, the story and motivation for everything was far more fleshed out

Here's just some of the things. Not even going into mechanics that are removed (like Wyll's eye). Or my personal opinion some of their starting looks were a bit more interesting/better (you get a bit of an idea https://www.youtube.com/watch?v=4MeutkQFliM or from the CohhCarnage videos I link later)

There used to be way more dialog options for one. Your class/subclass/god/etc actually got reflected in more options. There was a great video of a cleric giving some unhinged prayer to the tiefling that opens the gate. It's not major, but it adds to a lot to the game. https://www.youtube.com/watch?v=KameRUsFv8Y (with all the BG3 videos now, it's harder to find some of the classics). Also, Shadowheart had a much more interesting conversation if you were also a cleric if you were a Cleric of Shar too.

The camp was way more interesting. There's a ton of story ripped out of the game from the camps. This, was the thing that hooked me in EA. Daisy (the in-game code name for what would be replaced by your dream visitor) would entice you with power, encourage you to use the illithid powers. And when you finally do, and the narrator says "you've lost something you can never get back" probably makes more sense now. Once you did that, you got a class specific illithid power. There was no weird illithid skill tree, it was based around how often you used the powers. But going back to the camp, the companions would comment more about each other. Shadowheart and Lae'Zel's hatred for each other was shown way better, even commenting if you talked to one or the other first. Here's a couple of examples https://www.youtube.com/watch?v=gYRe2jHhBRc and https://www.youtube.com/watch?v=zw5q6u_iMN0 but overall, the camp at the end of the day was 1000x more interesting and really fleshed out the characters more and I'd say in important ways.

I'd even say the starting area (Nautiloid) was better. The current one is sufficient I guess. But the original showed you more about the fight the Illithids have with the Gith, you got to see more of the ship, what the Illithid did to people, what it meant to become a thrall (to really sell it that this is a terrifying process). But it showed you a few more mechanics too. https://www.youtube.com/watch?v=eC-GWA_Yj2c starts at EP2, but you'll want to go to EP 3 as well. But you can see the differences.

Tav used to talk.

For me, the story around "Daisy" was way more compelling. The song "Down by the river" is in reference to these encounters. Here's a video on that https://www.youtube.com/watch?v=pTw_9vM5LgA

... So these are just some of the things, lol. There's more like when you got down to the Underdark and some serious changes that happened there. And I haven't even touched on the stuff that never made it in (like the extra companion that was to be a werewolf... or the original Nightsong) (you can see some of it cataloged here https://bg3.wiki/wiki/Cut_and_unimplemented_content ) and I'm fairly certain all the companions were originally supposed to show up in Act 1 instead of 1, 2, and 3. There's a lot of dialog from characters for places they're not "supposed to be" like the eagles (can't find video, but its out there). But none of that is EA stuff, just cut stuff... but if the game had baked longer.... I'm just saying... it might not have been cut sutff.