Playing through Mario 64 for the first time has gotten me thinking a lot about level design. Not just how it works and what appeals to me, but how to frame those thoughts to discuss it with other people.
Mario 64's level design is tied very closely to its star objectives, and more broadly about coming to really know a 3D space very well. Knowing the layouts of platforms, understanding how your movement options interact with those layouts, and remembering the locations of different entities. Star 1 is usually "Walk around most of the level", the red coin challenge asks you to know the different spots of a level very intimately, and the 100 coin challenge to know the level very broadly. The last star is usually the most interesting in this way, since it expects you to have the most thorough understanding of the space and can use that understanding in interesting ways.
The first place that made me really slam my fist on the table and yell _Level Design!_ was Big Boo's Haunt. From my experience, the core of what makes this level feel the way it does is surprise. Boos, books, and fire emerge from unexpected locations. Falling down a pit leads not to death, but to a strange new area. The piano comes alive, Then, after going through most of the objectives in the level, you reach _Big Boo's Balcony_. If you explore the areas you've seen so far, you find nothing. If you look up at where a balcony might be from outside, you see a big boo way up somewhere you've never seen. It gives you the juicy design moment as a player of thinking "You can get up there? _How?_". Suddenly the ceilings and walls of every room become worthy of close inspection. Actually finding the door leading to the attic provides you with _the same juicy moment_! Up to this point, it's entirely possible that you haven't had to wall jump _or_ get partway through a triple jump to get anywhere (especially if, like me, you'd accidentally skipped Tall, Tall Mountain). Now you have a constrained space to really get to know your movement mechanics with the level design promise that "Yes, you _can_ get up there!".
Here's the thing, though. Most of what I've described is level geometry or entity positioning, and those are _far_ from all level design is. Texture placement is level design. Music transitions are level design. Where you put readable text is level design. Enemy behavior is level design. And there are many, many more ways to frame the discussion. The above has largely been about describing 3D exploratory platforming levels. You could use much of the same vocabulary to talk about Halo, but would you?
When you're talking to someone about level design, what is it that you talk about? Do you start at level geometry? Theming? What moments stand out to you enough to remember and want to talk about them, and how do you describe those moments?