One of the reasons why this poem is so incredibly well known and famous is the way that it is almost song-like in its rhythm and music. It can be compared to many songs because of its evocative rhythms, alliteration, rhymes and other sound effects that make it memorable. Remember that unless we hear poetry we are often not able to fully comprehend what the poet is really doing through the sound of the words and their rhythm.
In particular what is notable in this poem is the use of internal rhyme, or rhyme that occurs within the lines or repetition of an end rhyme within a line. For some examples of this, consider how "dreary" is rhymed with "weary" in line 1 and then "napping," "tapping," and "rapping" in lines 3 and 4. Likewise alliteration and onomatopoeia is combined in a somewhat exaggerated but definitely striking manner. Consider the following line:
What this grim, ungainly, ghastly, gaunt, and ominous bird of yore...
The repetition of the "g" sound which combines onomatopoeia with alliteration to great effects serves to reinforce the grim appearance of the raven.
All of these examples help to create this remarkable and intensely memorable poem through the production of what can be termed word-music, because so many effects are used that in many ways this poem is similar to a song.