Young Writers Society

I think that answer will differ a little from person to person, and while there are some standard things that maybe tend to make more people think a poem is "good" - these different factors won't be universal across all people, time, and space.

So with that little disclaimer, I personally think a poem is good if...
1) Clarity of Message Its meaning can be understood or discerned. (if I can't understand what the poet is trying to say, I'm not very likely to think it is "good')
2) Layered / Deep Meaning There are multiple levels of meaning within the poem (often done through use of allusion, or metaphor)
3) Creativity The language or approach is feels unique or creative.
4) Emotional Response The poem inspires emotional resonance and maybe even empathy with me as the reader.
5) Impact Is the poem saying something about something that feels "worthwhile".

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Other factors I find less important but often look at when reviewing - is the poem cohesive (does each line and theme feel like it belongs together), does the poem seem grammatically polished/intentional or messy with spelling mistakes and random punctuation, does it use poetic devices effectively (like assonance, metaphor, theme, repetition..), does the poem "sound good" to the ear, is there a sense of "story" within the poem (ie. do I feel like I'm watching a movie or living the poem, or simply being talked at).

That's just all my two cents! And sometimes the very best poems play with those different factors in new ways to create new meaning or new standards! Poetry is cool like that!