I've seen some very basic level designs, some extremely complicated, and everything in between. There are levels that I can't figure out at all (such as some of Richardfu's), and some that are obvious right off the bat. And then there are those levels that are so beautifully designed, such as VB's "Square One" - I have no idea how the mechanism works, especially in several stages like that, but it is very nice. Not easy to figure out, but not too difficult either. "Aztec Island" by B Hill (Benny Hill? Or am I the only one old enough to remember that?) with its ghost block... plus it's a silly design. Too many others to mention, but there are several nice ones out there. Thanks to tutorials from Trids and a couple of others, I'm finally starting to get the hang of the game. Eventually I might start designing levels of my own, but right now I'm having too much fun playing what's out there... and the mark of a good level is one that you want to play again. And speaking of levels, I like the community participation in levels - VB updated "Square One" twice because of comments and suggestions that people have made, and other authors, such as Moggies Race, have done the same. That's nice to see, real community spirit, and constructive criticism, literally.
Adding to the above... another level that I really like isn't complicated, nor is it difficult, and that is "Reinicio" by AndroidHackPro. It's straightforward and easy, with all of the draggables and tap spots in plain view and easy to get to, which isn't always the case in levels. But one of the nicest features of it is the fact that if B gets knocked off a platform or even tossed out of the level, he can get right back in via the stairs, which is a rare feature in any level. For example, "The Earthquake" by B Hill is an amazing level, nicely engineered and a genius concept - but if B gets tossed out of the level by that first moving block, there's no way for him to get back in, other than restarting the level; the only real 'flaw' of that level, as far as I can tell.
I agree - especially about providing a route back into the puzzle without having to reset it. Levels that I most admire are the compact, concentrated, minimalistic challenges. Especially where elements serve more than one purpose. To me, they're like engineering haiku. No fluff. Like @meko's "Walk Around", and @richardfu's "Return Trip"... amongst many others. Another kind of level that i prefer is where the element of luck plays no role. .. But in the end, as they say, "beauty is in the eye of the beholder"
And another thing that i really enjoy is when a puzzle teaches you something - like a new technique, or a new trick/exploit.
Wow, two mentions! . BTW it is good to use the @ sign before a profile name so the person gets an alert that there were mentioned. I just happened to be browsing the forum and saw this. I agree, it is a problem with the level, which is why I deliberately put it at the very beginning. But it is an earthquake and how many chances in a real life earthquake do you have for a redo? I also appreciate levels that don't make you redo the entire thing. And no I am not Benny Hill. B is a reference to B Bot.