1. One mistake is not even conceiving the poem in poetic terms at all. This is the poetry of bald statement, with not even a gesture in the direction of poetry. The poem will sound like it was written by someone who has never read a poem at all. This can be a good thing, ultimately, as there will nothing to unlearn.
2. Similes. So everyone's favorite poetic device is the simile. The problem is that it sounds very junior high school, most of the time. It tends to signal "this is a poem" in a kind of obvious way. We know that metaphor is stronger: can we imagine Manrique saying"nuestras vidas son como los ríos." An intermediate level move is to suppress the "like." A simile can be great if it is not a throwaway poetic device: "I love you as a sheriff searches for a walnut..." (Koch). That is a great one.
3. Stereotypically lyrical language. A lot of shimmering evanescence, and words suggesting that mood, like shimmering or evanescent, will also tell the reader you are a beginner. Why? Because those words a short-cut toward lyricism.
4. Clichés. You simply cannot use clichés in poetry, except ironically and even then only if your name is John Ashbery. Tires must not squeal, you cannot have people glued to the television.
5. Present tense. If your poem is in the present tense and you describe yourself doing various actions, then you are incurring in a discursive cliché. Only do this if you find some brilliantly original way to do it, like Frank O'Hara.
6. Inept enjambments. Enjambment often looks random in poetry, so beginners naturally think line breaks are just arbitrary. Actually, they provide a structure of revelation or unfolding, and highlight rhythmic structures.
7. Lack of prosodic intentionality. Free verse is free, so anything goes, right? But good poets actually think about sound and rhythm. If the prose rhythm (the rhythm the words would have if they weren't broken into lines) still predominates, then the reader will know you how just written the poem first and broken it into lines.
8. Period clichés. These are things that you have picked up from other poets of your time, that you see a lot of other poets doing. This is what makes your poem sound like any other poem that anyone might write. Your poetry might be more publishable with more period clichés, but it will not be better.