The heart of Mario 64 - to me, anyway - was the castle itself and the castle grounds. I could - and still do - have a blast just running around and exploring there.
Mario Odyssey, meanwhile, is basically just a straight-line path.
But once you got into the level in 64 you were put on a linear path to get said star. Odyessey opens that up, and doesn't kick you out of the world every time you complete an objective. Each kingdom in Odyessey feels like a castle in it's own right that you can explore to your hearts content. Each kingdom also has dynamic changes that occur from the first time you enter it to the time you leave. There may be linear progression in the flight of the odyessey but this is by far the most freedom we've had in a Mario game.
Yeah, if you go off the beaten path, very often I find some very cool puzzles, platforming challenges, etc.
And yeah I feel like you're looking at Mario 64 with rose-colored glasses (and it's one of my favorite games of all time). That game much more linear outside of a few stars you can collect whenever and there's definitely more reward for exploring here in Odyssey (either coins, stars, region specific coins, extra hearts, etc) if you think something might be there. The earlier kingdoms also seem to open up more the farther you progress. New Donk City is probably where the game really starting to feel non-linear I guess.
I'm, idk, maybe between 2-3 hours in, mostly loving it of course. 95% of my Switch play has been docked with pro controller, but I've played exclusively with split joycons so far as that's the suggested setup or whatever. The movement controls clicked right away and feel great except for one thing -- the cap spin move. That motion of jerking both joycons to the left is just......not good? It feels awkward and terrible every time to me. Instead of that I've been just throwing the cap and then kinda waving the joycons wildly and it tends to just spin after a bit of flying around lol. But anyway, next time I boot the game up I'm gonna try with pro controller and see which setup I prefer.
I'm in the third kingdom, and if anything I actually think the game has been way more linear so far than anyone let on. It's still entirely "do this. now do this. now,...do THIS" so far which is kind of ...eh. I mean I know I'm still very early, but I'm looking forward to hopefully having the shackles totally thrown off sooner rather than later, but maybe Breath Of The Wild conditioned me to expect too much freedom in that regard.
Game is freaking gorgeous, music is fantastic, etc.
Yeah the first 3 kingdoms seem kinda linear but they open up more later, and I know I found a ton of additional moons by exploring a lot.