What makes a poem good?

Actually, first and foremost to consider when you read a sonnet is whether it should be a sonnet or not -- the same goes with any form of poetry including free verse.  The one thing that is guaranteed to make me instantly dislike a poem is if it seems jammed into the wrong shape -- unfortunately, to recognise when that shape is wrong requires knowing what it ought to look like, and that means reading a lot of poetry.  Though it's not entirely possible to say what makes good poetry, it is certainly possible to say what makes a poem a really bad example of what it's claiming to be.  It just doesn't fit.