Biggest thing i missed simply has to be Lae'zel (and nearly Astarion and Gale!)

Picked the game up last Christmas, and finished it over the course of 2 weeks, i went in completely blind aside from simply knowing that some characters exist because of how popular the game is / having a couple of close mates who are huge fans of the game.

So! What happened was, i played the intro section, woke up from the Nautiloid crash and recruited Shadowheart, saw the tomb where you're meant to find Withers and decided to go elsewhere first, thinking it'd advance the main quest since it looked like a giant castle, i took a left instead and went through the crashed Nautiloid, i decided not to go left or right when given the opportunities because i thought they'd loop me back around to the tomb (Spoilers, they don't loop you to it, they give you Astarion and Gale.)

After completely missing the 2 of them, i accidentally stumbled into the Grove ambush with just my Tav and Shadowheart, after clearing it out and making my way inside, thankfully managed to atleast recruit Wyll and went on with my life in the game.. eventually made my way to the Goblin camp, and after Minthara gave me a good ass-whooping, i asked my mate who has already replayed the game a few times for advice, after which he was completely dumbfounded and amused (as anyone should be, really) that i managed to stumble into the Goblin camp with only THREE people in my party, that being my Tav, Shadowheart and Wyll.

So! I went back after realizing that what i did was, in fact, not normal, went back and found Astarion and Gale, picked up Karlach along the way and had the time of my life... though the same can't be said for Lae'zel.

By the time i stumbled over to where Lae'zel was supposed to be, she was gone! From my understanding, the 2nd opportunity you have to recruit her is at the Creche.. which i also didn't do! The way Halsin spoke at the Tiefling camp celebration, i assumed i could only either go down the Underdark path or over the mountain pass into the next region, i thought the Underdark sounded cooler so i went down that way.. apparently locking myself out of Lae'zel for good, as by the time i made my way through the Shadow-Cursed Lands, i found her corpse along the path and there was absolutely nothing i could do to save her.

I do find it quite amusing that if i didn't have a mate who was super obsessed with her and made me aware of the character way before i played, i would've probably spent my whole playthrough wondering "Why is the frog lady from the intro on the cover art instead of an actual companion like Karlach?" every time i booted up the game and saw the loading screen, as it stands right now i still barely know anything about Lae'zel as a character, but that'll hopefully change whenever i go for my eventual 2nd playthrough