It seems like the difficultly level of the different levels actually matches pretty closely with the list of how many pieces it takes to solve a level.
I'm not advocating the order be changed or anything. Many users will figure out that they can approach the levels in any order they want anyway. I just found it interesting how closely that the list was starting to match my perception when I played through the first time. Some levels shocked me with their appearance, but after the initial "what the heck" moment it turns out often that a solution wasn't hard to come up with or implement. Meanwhile some levels like Awash are deceptively hard problems.
I'm going to sort by the pieces to solve as they currently stand and give the benefit of the doubt to the original order on ties:
1. Learn to Play - 0 Pieces (orig. #1)
2. Easy As Pie - 0 Pieces (orig. #2)
3. Rolling Away - 0 Pieces (orig. #3)
4. Around the Bend - 0 Pieces (orig. #11)
5. Reach Up - 1 Piece (orig. #4)
6. Mind the Gap - 1 Piece (orig. #5)
7. On a Roll - 1 Piece (orig. #8)
8. Mission to Mars - 1 Piece (orig. #18)
9. The Wall - 2 Pieces (orig. #7)
10. Full-Up - 2 Pieces (orig. #9)
11. Higher - 2 Pieces (orig. #10)
12. Junkyard - 3 Pieces (orig. #6)
13. Up the Hump - 3 Pieces (orig. #12)
14. Up the Stairs - 3 Pieces (orig. #14)
15. Big Ball - 3 pieces (orig. #15)
16. Down Under - 3 Pieces (orig. #17)
17. Handling - 3 Pieces (orig. #19)
18. Tube - 3 Pieces (orig. #20)
19. Four Balls - 5 Pieces (orig. #16)
20. Back and Forth - 5 Pieces (orig. #21)
21. Awash - 6 Pieces (orig. #13)
22. Unpossible - 8 pieces (orig. #22)
I think thats quite close to how difficult the levels actually are.
Awash in particular is a very challenging level, I think second hardest may be appropriate. Without question its harder than Up the Stairs, Big Ball, Down Under, and Handling. There just aren't any "aha" tricks to make it easy. That avalanche of different sized balls has an amazing ability to find the weakness of any machine and either pin it down or send it to oblivion. It refuses to be brute forced, and requires at least some cleverness. You can't just build a dozer with regular wheels and chug up the slope the first time. I'm almost certain it took me more attempts to solve it the first time than any other level. I also love the look of it, its aesthetically pleasing and quietly says to me "whatever you have in mind, I will crush it". Maybe thats just my experience talking.
Junkyard may be out of order in the list above. Its a rather easy puzzle but it at requires at least a Club or a Car, which are both 3 pieces minimum. Its probably the biggest exception on this list in my opinion, since I don't think it is very hard to come up with a car design that solves it. Of course, now that I'm familiar with the Club I wouldn't go any other way.
Around the Bend may be out of order too, but it probably just needs to be fixed so there isn't an orange ball you can cheat off of to start with. If it weren't there, it would probably be a 2 or 3 piece solution, although a very cleverly placed stick might get it to one. I may have to try that now... Confirmed, it can be solved with one stick even if the orange ball weren't in there. If you're mean/clever, you'll switch the orange ball to be out of the start box but in the way on the right of the platform. Its pretty darn easy as is. #7 without the orange ball seems about right, coming just after On a Roll.
Mission to Mars might benefit greatly from having the goal be at least a quarter as much higher on the map than the start box. I think it would slip much further down into the harder category in that case. As it is now, I agree with the post in the other thread that even though it may look hard at first, in the end it turns out to be quite easy.
I'll be curious to see what others think.
Let me also take the chance here to say that I really enjoy this game. Its great! [Edit]I'll have to go in and find the link you mentioned.