As a whole, "I Am" sounds a lot like the musings of a sad and lonely old man, but an old man who knows how make a point without whining. Take the first lines of the first stanza for example: "what I am none cares or knows ; / My friends forsake me like a memory lost" (1-2). The short sentences get down to brass tacks right away, while the alliteration in "friends forsake" emphasizes what the real problem is without wasting any time. That phrase, "friends forsake," really sticks in your ear after you've read the poem.
As we finish the first stanza and get into the second, the poem starts to sound more like rambling, but rambling with a point. Notice, for example, that the last line of the first stanza begins a sentence that doesn't actually end until the last line of the second stanza. The continual addition of clauses to this sentence makes the poem sound like the ravings of a man whose mind is either starting to go, or who has a lot to complain about.
In the first two stanzas, the speaker really wants to drive home the fact that he's still alive. If you glance at these stanzas you'll notice a preponderance of long vowels: O ("woes," "host," "throes"), E ("me," "esteems," "sea," "dreams," "neither,"), I ("I," "like," "rise," "stifled," "life"), and A ("strange," "stranger"). Long vowels take longer to pronounce than short vowels; they seem alive, while short vowels seem "dead," cut off, lifeless.
So, the first stanzas are similar, got it. In the third stanza, however, things change a little bit. Here, one can't help feeling that we're listening to a prayer. All that stuff about living with God and sleeping peacefully sounds like something you might hear in church: "There to abide with my Creator, God ; / And sleep as I in childhood sweetly slept."
The other thing to note about the final stanza is how much "quieter" and more peaceful it is. The sibilance, repetition of the S ("smiled," "sleep," "sweetly slept," "grass"), in particular contributes to this effect. It is a quiet consonant, and gives the final six lines a soothing, almost dreamy quality that perfectly mimics the death the speaker desires.