The first words that the titular character speaks in Shakespeare's Hamlet are spoken as an aside (a character's thoughts spoken aloud), which the audience hears but the other characters on stage do not hear. In Shakespeare's plays, characters speak the truth in asides and soliloquies for the simple reason that they're speaking to themselves and therefore there's no reason to lie.
With Hamlet's first words, "A little more than kin, and less than kind" (1.2.67), Shakespeare takes the audience directly into Hamlet's confidence and into his life. Shakespeare puts Hamlet in the forefront of the audience's collective mind. From that moment, Hamlet is the primary focus of the audience's attention. Everything that occurs in the play is seen through Hamlet's eyes and is filtered through what the audience believes are Hamlet's sensibilities. It's truly remarkable that Shakespeare accomplishes all of that with a single line of iambic pentameter.
It's important to look at Hamlet's first line in the context in which it occurs and as the first and only words that the audience has heard Hamlet speak so far (not in hindsight, with knowledge of the entire play).
At this point in the play, the audience knows nothing about Hamlet's scene with the Ghost ("O my prophetic soul! "), or Ophelia ("Get thee to a nunnery"), or the Players "Speak the speech"), or Hamlet's duel with Laertes ("A hit, a very palpable hit"), or Hamlet's last line: "The rest is silence."
The audience hasn't yet heard any of Hamlet's soliloquies, in which he exposes and explores his innermost thoughts and through which the audience learns more and more about him. The audience hasn't even heard Hamlet's pun in his next line about being "too much i' the sun" (1.2.69).
The only words that the audience has heard Hamlet speak so far are "A little more than kin, and less than kind," in response to a line from Claudius.
CLAUDIUS: ... But now, my cousin Hamlet, and my son (1.2.66)
Considering that the audience knows almost nothing about Hamlet (his name has been spoken only once thus far in the play, in the preceding scene, and this is the first time he appears on stage), this first line needs to be taken purely at face value. In fact, someone in the audience who has never seen or read Hamlet and knows nothing about the play might not even know which of the characters on stage is Hamlet until Claudius addresses him:
A little more than kin
First of all, who is "a little more than kin"? To whom is Hamlet referring? Claudius, himself, or both? In what way(s) is Claudius, or Hamlet, or both "a little more than kin"? Do Hamlet and Claudius have more than a kinship relationship?
Claudius addresses Hamlet as "my cousin" and "my son." It might be unclear to some members of the audience what the relationship actually is between Claudius and Hamlet. Is Hamlet Claudius's cousin? His son? If not, why does he call him "my cousin" or "my son"?
and less than kind.
Again, who is "less than kind"? "Less than kind" in what way(s)? "Less than kind" in terms of actual kindness towards one another? In terms of "he's not my kind"? In terms of "we're not at all alike"? Or all the above?
All the audience knows about Hamlet from his first line is that they still don't know much about him, except that he can turn a clever phrase, even though the meaning of the phrase is wholly unclear.
Essentially, the audience knows very little more about Hamlet after he speaks his first line than before he spoke his first line.
The audience simply needs more information, but thanks to Shakespeare's skill as a playwright, the audience is immediately drawn to this enigmatic character and into his life, and the audience is going to spend the next four hours watching the play to find out as much as they can about him.