Based on context I assume they mean in regard to setting and broad strokes mechanics

If you want DND with working rules, Pathfinder 2e is what people recommend. Not first edition. I'm not a huge fan since it's still basically DND.

If you want a lightweight system that's mostly about narrative, I'm a fan of Fate. But Fate is absolutely not a crunchy system, and it's largely up to the group to agree on what makes sense. Like, if you want character differentiation you can lean on "aspects as permission" and it's right there. (That is, stuff that's true about your character permits you to try stuff. The barbarian can't even try to decipher the runes, because nothing about his character implies he could do that. You can't just blindly roll something. The wizard can try, because of course wizards know runes)

The core rules are free, but you can find books with more specifics. I think there's a Dresden files book people like? They don't provide a complex magic system in the core books, but it has some ideas and the toolkit book has more.

I also liked the chronicles of darkness games, but they're generally all modern day occult. You can take the core rules and move them to fantasy, if you wanted. It's pretty light and I like it more than DND in all the ways I care about.

Blades in the dark and other forged in the dark systems are famous for being an early breakout in RPG game design. I've run a blades in the dark campaign and it was awesome.

Check out the YouTube channel "Quinns Quest" and I'm confident you can comfortably try any game he has reviewed to get a better roleplaying experience than D&D.

Daggerheart. 🤘🏼

If you want some of that DnDness, I really liked 13th age, and Fanatsycarft. They are on opposite sides of the complexity spectrum. I liked those more than either PF. Ryutama taught me a lot of things about RPGs but a half assed combat system means I can't recommend it., especially for a longer campaign. However if you're interested in something on the experimental side, then I think its perfectly lovely for a one shot.

Daggerheart is new, and Im not particularly interested in CR, so I haven't really checked out Daggerheart beyond a cursory scan of the PDF.

I enjoyed the heck out of Icon. Exalted is also fun for your fantasy needs. Its a different flavor from dnd though, both mechanically and the kinds of stories they want you to tell. If you're looking for more flexible, open ended stories, fantasy or otherwise, I really liked Fate Accelerated.

Thats just if youre looking on the fantasy side of things. On the Sci fi side, Ive been playing Cyberpunk Red and really liking it (although fatigue has yet to set in.) I really liked Shadowrun 4e, and I recommend that to anyone who wants to dip into that (Karmagen optional rule and Chummer.exe are you best friends). Lancer is a heck of a thing.

I'm a GURPS fanboy. Complexity is determined entirely by how many rules the GM and players agree to - I've got a player with dozens of skills and several wonky abilities who plays side by side with an 8-skill player. They both do well, and both have what they want - one character is just more detailed than the other.

The trick with GURPS is to pull in the bare minimum of rules that you need for the kind of game you want. If you don't like how a rule works? Swap it out!

I've enjoyed the Ironsworn line too, for a different feel. That's a much less complicated system that can also be played GM-less, it works narrative elements into the engine - the story dictates how the rules apply!