Good level design depends on the kind of game you wanna make. Level design is part of the overall "gameplay", which in turn is the sum of all the actions the player is doing/the developer(s) wants them doing. Level design, therefore, is the sum off all the visual elements the developer creates (whether that be 2D or 3D) that forms a certain "playground" for the player to perform actions in. Now... GOOD levels design is it, if the developer achieves to make that "playground" interesting, immersive, exciting, challenging, audio-visually pleasing and overall "FUN" to "PLAY", all within the context of the overall game design/game concept. GOOD level design helpes to make the actions of the player feel enjoyable during the whole gaming experience. However, there can be many different types of game design/game concepts that all represent a different kind of "enjoyment", e.g. the whole philisophy of Dark Souls is to be hard and keep the players on their toes - so that kind of difficulty IS the particular enjoyment this games wants the players to feel -, while, on the other hand, a diving simulator like Blue Ocean on the Wii wants the players to explore it's world in a relaxed manner and make them feel enjoyment this way. That's why good level design depends on the games design/concept and what kind of enjoyment the game tries to create.
TD;LR: Good level design is the sum off all the created ideas that help the gameplay/game concept of a game to achieve its particular style of enjoyment.
Example "2D Platformes": their particular style of enjoyment/goal is to make the player have "fun jumping from left to right". So, good level design is the sum of all the created ideas that help the player to have "fun jumping from left to right" (that includes stuff like placement of platforms, enemies, items, collectables, secrets, surprises, etc. - so in short, all things/ideas that help the overall game's style of enjoyment/goal)
Therefore, good level design can only be judged in the context of a games overall design and what kind of enjoyment this game tries to create. Yet, this makes it also a bit tricky to judge. It goes without saying that good level design, however, doesn't depend on a players personal preference, just like good gameplay doesn't depend on a players personal preference.