There are a ton of plants in the poem, but this is obviously the most important. After all, it's the thing that drives the entire plot. This plant—with its strange, almost magical effects—is a source of conflict, but also of all the odd, beautiful, and surprising things that the sailors have to say. It's dangerous, for sure, but also kind of tempting, and it's that contradiction that makes this poem exciting and interesting.
Lines 28-29: This is the first description of the Lotos itself. And actually we don't hear that much about it. What we do hear is tantalizing, though, including the magical, mysterious reference to it being "enchanted." Notice how Tennyson amps up that magical feeling with alliteration: "branches they bore" and "flowers and fruit."
Line 105: The little repetition in this line is subtle, but also important. By repeating the word "day" the sailors hint at the endless, unchanging future they are dreaming about. They are sick of excitement, and the Lotos is a gateway to a world that never changes.
Lines 145-146: Okay, humor us here: just say these lines out loud a couple of times. Can you hear how rich and beautiful the sounds are? Tennyson is the king of this stuff. The Lotos is magical, so he describes it with magical sounding words that float and dance off your tongue. That effect comes partly from alliteration (listen to all the B sounds in these lines). It also comes from assonance, the repetition of the O sounds in "bloom," "below," and "blow."