I caught up on this a bit late today due to a prior conflict, but I've pored over the video and read through this thread, and I am just so overwhelmed.
Still, a few things I want to talk about that people don't seem to be discussing at all:
1. Scroll stop. Nobody's talking about this! Do you know what a big deal this is? On my own courses I had to leave considerable space to isolate areas and keep them out of view, as did anyone who used the old horizontal layout to create pseudo-vertical climbing courses (which I'm sure will still be around if the new vertical layouts are restricted to sub-zones) or multi-room puzzle stages. But the biggest impact of all will be on control over despawning distances. It will now be possible to build boss arenas or contraption rooms wider because you can set a hard scrolling boundary on either side. Despawning bugs were always a massive pain for ambitious designs with lots of moving parts, and this considerably enhances the toolkit for keeping them under control.
2. Post-completion notes and comments. Not that random strangers ever play anything twice, but on the "Post a Comment" screen, notice the checkbox labelled "Require course clear to view". I'm elated to have comments/annotations back (along with a creator description that you can use to give a hint, synopsis, or optional objective right from the course selection screen), because of the power it gives you to enhance your own creations by communicating unconventional mechanics or introducing narrative material. But this adds a whole other level: it allows players to comment on particulars without spoiling a solution for newcomers, and if you're annotating your own work as a course creator, you could even include something like a "developer commentary" for returning players.
3. Dry Bones Shell. Since Mario wears this below instead of above, in the same manner as Kuribo's Shoe, I wonder if it will be possible to wear the Dry Bones shell on the bottom and a Spiny/Buzzy hat on top (again, like the existing combinations with the boot) and what the priority will be when you take damage (currently, the boot goes first). This is one of the first things I want to test with the new item. I also wonder if its interaction with lava is specific to the Castle theme (where lava replaces bottomless pits) or if it will also work as a flotation device on water stages. It looks like it works with rising and falling lava, though, so I don't see why its interaction with water wouldn't be the same.
4. Clear conditions look optional. Or are they? Can you still reach the goal the standard way without them, and does it count? If so, an open question: are they separately tracked or timed, and will we be able to see a separate completion rate for them? I'm excited to play with these, because if they are optional, they essentially create a built-in hard mode for every single course you build (a no-hit run, for instance), without having to take up two upload spots. Conditions like "reach the goal with this item" finally give us something to do with pink coin objectives scattered throughout the course that unlock a bonus door at the end. And although we haven't seen it yet, the condition I really want to see here is a no-checkpoint run.
5. Search by difficulty. Notice how there's a difficulty selection and a "sort by clear rate". Speculatively, by using both of these at once, you could look for something tagged as Expert (for instance) and sort by clear rate, lowest first, to pick courses of an appropriate difficulty for yourself that still filter out Super Expert. In SMM1, the Easy/Normal/Expert/Super Expert designation actually isn't based on clear rate, as far as I can tell. Clear rate is based on "clears per total attempts" (successful clears over total lives); from what I've observed, the difficulty designation is based instead on "percentage of players cleared" (successful players over total players). I personally don't know if I'll get that much use of the in-game discovery features, as I always found in SMM1 that the very best experience came from browsing communities and recommendations outside of the game, but I think the fine-tuning in SMM2 could produce some quality results.
These are just some scrappy initial impressions; I'll jot more down as I think of them.