"The Wild Iris" uses a lot of simple words, including many that contain just one syllable ("there was a door […] Hear me out […] I tell you I could speak"). These are the types of words used in ordinary speech, and they make for a conversational style in the poem. Louise Glück's poetic style is often called "spare," meaning plain. Turns out she doesn't like fussy clothes either. According to the Yale Daily News that she is often seen wearing a no-nonsense baseball cap and her trademark leather jacket—how cool is that? (source).
Still, Glück's poems are not easy just because they tend to use simple words. And she doesn't rely completely on simple vocabulary. When she does introduce a fancier word ("oblivion … azure"), it packs a wallop, contrasting with the unremarkable words that surround it.
Plus, the ideas in her poems are often deep and complex. "From the beginning I preferred the simplest vocabulary," she explains. "What fascinated me were the possibilities of context […] I loved those poems that seemed so small on the page but that swelled in the mind" (source).