A good challenge is providing an obstacle the player wants to overcome, no matter how hard it may be, if the obstacle is of interest to the player it will keep him playing (You could also add some ploy to decive the player into thinking the obstacle is relatively easy and if he can't defeat it he sucks ;P). Even if the obstacle doesn't appeal to your tastes, it's common sense to tell what you simply don't like and what is just plain absurd.

Of course even boring obstacles will keep you playing if there's a story or some other motive keeping you. The 'newbie' levels you're describing probably didn't have a story or one that had your interest, so you focus more on the flaws and boring stuff. I also get the impression that some authors focus more on just trying to earn praise and ratings but wouldn't enjoy playing their own levels. The "DO YOU JUST LIKE EASY LEVELS" Quote sounded like he was trying to make the gameplay as hard as possible, since he probably read a bunch of reviews complaining about how certain other SP packs are too easy.

Level design is ultimately not about following any standards, you should make what you want to play in. Most of which you would want to play is probably what other people want to play as well.

My 2 cents.