Controversial opinions ahead as always. Consider this your "trigger warning" =P
In the context of helping n00bz:
A "good review" is one which helps the composer improve and achieve their goals for the piece or the next piece. I would say that's all there is to it on what makes a "good" critique. This is entirely context-dependent. People can leave walls of text that are totally useless and one guy can drop a one-liner about how "you need smoother voice-leading to make these chords work; move each note only the shortest distance possible to the nearest chord tone" and that could be more helpful in improving the piece than anything anyone else wrote.
However, and this is the biggest problem frankly with internet feedback: Many people, also being noobs themselves, can recognize that something is off, but they often do not have the knowledge and skill themselves to tell you how to, using music theory instead of abstract/subjective descriptions, to adequately solve problems, which in most novice compositions and recordings, are simply too numerous to address in detail in a forum post, and most experienced composers are not going to put in that kind of time investment into a stranger's music on the internet — harsh but true. It's mostly going to be n00bz giving you feedback and this has really quite limited utility.
Ending on a related note: The above is why I recommend people don't rely much on forums and such to improve. If you compare your music to music which is better, and then study that music, the relevant theory that went into it, etc. top to bottom, imitate, and so on — you will keep getting better provided you're making a serious effort at it.
Most great songwriters and composers from at least the late 20th-Century onwards had little outside critique on their music unless they had a mentor, and many didn't even have that. Most were just aesthetes and perfectionists who never stopped never stopping at picking apart their own music against music they admired until theirs was sounding at least as good, and in time better than their heroes.