it's only fair
When you pick up a book of poetry, you read the poems, form opinions about each of them, maybe even highlight certain passages that strike you. When you do, you don't for one minute expect that author to read your work do you? Of course not! Especially since so much of what we read was written by poets who just happen to be dead now!
But, you see, this isn't the case with forum board poetry. Reciprocity is what makes forum board work -- it's what makes our little poetry world go 'round.
Every poet who posts a poem on a poetry forum board expects to be read and replied to, and the main way we insure that will happen is to read and reply to other's poetry. It's impolite to do otherwise, to expect to receive replies without having given any is rude and selfish.
The point being: If someone has read your poem and made a reply, you should return the favor -- read their work and offer your insights and opinions. It doesn't matter how much technical knowledge you posess, or how it compares to how much they have. What matters is that you are polite and considerate enough to read them, and tell them whether or not you enjoyed their writing, because it's very discouraging to write and post, and feel as though no one bothers to read what you've written.
Now in my opinion, the more in-depth their comments are, the more thought you should put into yours -- but really what matters is that you don't leave a fellow writer who's taken time to read your work out in the cold feeling as though he or she's been used.