• that he feels himself to be powerless and undeserving of respect.
• "He would drown the stage with tears." Nobody's tears are sufficient to drown a stage. What Hamlet is saying is that if the actor "had the motive and the cue for passion" that Hamlet himself does—rage and horror at his father's murder—the actor would be able to express it through copious weeping, something Hamlet himself cannot do.
• "I am pigeon-liver'd and lack gall." Hamlet does not have a pigeon's liver; he is comparing himself to a dove. In Shakespeare's time, the liver was thought to be the seat of emotions (rather than the heart). By saying he has a pigeon's (a dove's) liver, Hamlet is referring to the supposed gentle nature of doves and implying that he is likewise timid and gentle, too much so to do what is necessary and avenge his father's death.

Synecdoche is "A figure of speech in which a part is made to represent the whole or vice versa [2]." For instance, when discussing World War II, a person might say "Germany fought Britain," which means that the German military fought the British military; it's understood that the names of the countries stand in for the acting parties.

Hamlet has a few instances of synecdoche in this passage:

• "[The actor would] cleave the general ear with horrid speech." The "general ear" stands for the listening faculty of the audience. If the actor could express Hamlet's feelings on his behalf, the actor would horrify anyone who listened with what he had to say.
• "Murder, though it have no tongue, will speak / With most miraculous organ." The "miraculous organ" stands for the play Hamlet will instruct the actors to perform. This play will depict the murder of his father, thus allowing the murder to "speak" to the audience and be known.

Imagery is "Visually descriptive or figurative language, especially in a literary work.[3]" It appeals to the senses, and makes the reader see, hear, or feel what is being described. Shakespeare makes heavy use of imagery in much of his language, which is part of what makes his work so evocative.

The imagery Hamlet uses in his speech includes (but is not limited to):

• "[His] visage wann'd, / Tears in his eyes, distraction in's aspect, A broken voice [...]" Hamlet is describing the actor's convincing portrayal of grief.
• "Who calls me villain? breaks my pate across? / Plucks off my beard, and blows it in my face? Tweaks me by the nose?" Hamlet, having asked himself if he is a coward, is describing how cowards are and should be treated, by being abused and insulted by their peers.

A simile is "A figure of speech involving the comparison of one thing with another thing of a different kind, used to make a description more emphatic or vivid [4]." Similes use the words "like" and "as," for instance "free as a bird" or "hungry like the wolf."

Much of Hamlet's imagery is present in his similes, which include:

• "I . . . like John-a-dreams." John-a-dreams is just a term meaning someone dreamy and inattentive, a daydreamer. By saying he is like "John-a-dreams," Hamlet evokes a picture of himself as scatterbrained and distracted.
• "I . . . Must, like a whore, unpack my heart with words, / and fall a-cursing, like a very drab, /A scullion!" Hamlet knows he needs to avenge his father's death but is finding it extremely hard to take action. He is furious with himself for prevaricating, and constantly arguing with himself "like a whore," "a very drab," "a scullion"—that is, someone immoral, of low breeding, and undeserving of respect.

There is always more to "unpack" in any passage of Shakespeare, so now that you have a sense of what you're looking for, I'd invite you to go back through Hamlet's speech and see if you can identify any more instances of these four literary devices.

[1] Oxford Dictionaries definition of metaphor
[2] Oxford Dictionaries definition of synecdoche
[3] Oxford Dictionaries definition of imagery
[4] Oxford Dictionaries definition of simile

a) Metaphors are comparisons that do not use the word "like" or "as."  Some examples:

"O, what a rogue and peasant slave am I."  Hamlet is not really a slave; in fact, he is a prince.  He means to say that he is acting like a peasant slave who lacks courage. 

 "Why, what an ass am I!"  Hamlet is not really a donkey.  He means to say that he is acting foolishly, like a donkey.

b) Images.  The two metaphors above can also be considered examples of imagery; they cause us to picture slaves and jackasses in our minds.  Other images are:

"drown the stage with tears";

"I should have fatted all the region kites/With this slave's offal" (I should have fed all the hunting birds in the region
With my body’s decaying flesh);

c) Similes are comparisons that use the word "like" or "as."  Some examples:

"Yet I...peak like a John-a-dreams" (shrink from my task of revenging my father's murder like a dreamy fellow);

"I...must...fall down cursing like a very drab, / Menial servant in the kitchen."

d) A synechdoche is a figure of speech in which a part is used to represent the whole (abc for alphabet) or the whole represents a part (Boston won the World Series).  The term is also used for other substitutions, such as the object contained for the container ("I was injured by some pickles" rather than "I was injured by a can of pickles), and the action instead of the actor.

This passage contains an example of the last type of synechdoche:

Because murder, although it is silent, will speak
With its own most miraculous organ.

Hamlet does not mean that "murder" can speak.  Rather he means that the person who committed the murder, Claudius, can be made to speak by having him witness a play that is "something like the murder" of Hamlet's father.