I didn't get a chance to leave feedback on the first round, so here are some armchair-designer impressions from Round 2.
Boba2007: 1-1 Koopa Field
Simple and effective for a "1-1", but nothing stands-out. Nothing bad either. For a stage named after a specific enemy type it would have been nice/interesting to see slightly more clever/creative usages of Koopas, too.
Chopper: 1-1, A Bit of Fun!
Simple and effective for a "1-1", (maybe a bit more advanced with the varying enemy types as compared to the stage above) but nothing stands-out here either. There are some well placed hidden blocks in this stage, however, as my natural instincts to jump in certain hot-spots (such as that lowered mushroom stalk) were clearly rewarded. Stage is appropriately named though, as it was a bit of fun.
Cam: Mario & Luigi: Partners in Grime
There's a decent idea/concept at play here, but it doesn't seem executed too well. A part of that reason could be due to the limitations of Mario Maker, however. Switching between Mario & Luigi should showcase new/different ideas. As it is, it's a few screens with simple platforming ideas that either character could perform. The gimmick falls flat when you go back into a door you just came through, too. My take-away is that this level gave me some ideas for a similarly designed stage, so that's something!
Azure: 1-1 Summer Stroll
Simple, short and effective for a "1-1". There's a decent spike helmet gimmick at play here that shows elements of layered progression (ice blocks in-the-way, head back, get helmet, destroy ice blocks, "?" item blocks now have breakable blocks underneath them which makes use of the power-up, etc.). I like the safety net with the donut platforms near the final few moving platform jumps, too. Stage seemed actively aware that it was an intro level. Stage name is kind of weird, though. Any reason in particular for the title? Doesn't really seem like a stroll. Could have worked better to have placed some long stretches of background scenery to capture the feeling of a "stroll".
GAFBelgian: Super Climber Bros. 1-1
This was kind of a weird stage. This level was essentially a collection of training houses/rooms for Mario's move-set, so it served its purpose as a "1-1" level. They weren't mandatory either, so you could by-pass all of them if you already knew how to play which is cool. I appreciate some of the aesthetic touches, too (rounding your block edges, using backgrounds for the inside of buildings and I like the little fire jets that acted as chimneys on a roof). The level could have used a little more room to breathe, however, as I feel like NSMBU doesn't lend itself too well to tight-spaces. Again, what's up with the stage name? I would have titled this stage, "House of Training" or something along those lines.
Kebinu: Koopa Trails
This was probably my favorite stage so far, aesthetically. The stage was very pleasing to the eye (object placement and not allowing the player to see outside of the borders when in the caves/pipes, for example). Seemed finely tuned and crafted for being a "1-1", too. Good usage of hidden blocks and I dug the bonus pipe that taught the player that they could break the blocks with their leaf power-up. Any reason for the stage title? Didn't seem overly focused on Koopas.
This is all the feedback I have for now. I'll try to post some impressions for the rest latter.