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What makes Maus powerful is the way Art Spiegelman forms the narrative keeping it's integrity, with a mixture of Vladeks retelling, the emotional comic insert, and the inclusion of his contemporary family issues while creating the book. 
Spiegelman doesn't imbue any introspection in his characters; instead he uses his comic as a recording device, inscribing the characters' dialogue leaving the reader as a judge.  This is common in the comic book medium.  They usually don't talk a lot about feelings and inner perceptions, often these are rather obvious to pick up in the superhero genre of comics like Superman or Spiderman, but in Spiegelman, the complicated issues cannot be decoded easily, and they should not.  As readers we get a chance to try to understand Vladek in a way Spiegelman did not paint him.  The only trace of "inner thought" are the literal thought bubbles, limited to the space of a few lines unlike novels which can go on for three pages describing an emotional reaction.  Although Art could have described Vladek's life in the third person narrative, he goes one step further by using his own perspective describing the father's act of telling of the story, among other contemporary information.  This extra information gives us a better idea of the characters behind the book.
Secondly, Vladek's retelling pretty real, at least as much Vladek thought so.  There are mistakes in the chronology, something a first time storyteller always does.  By keeping them, Art gives the comic the flow of a running notebook.  Since Vladek reveals embarrassing bits of information about himself to his son, things that he might have left out.  For example, Vladek could have started his story after he and Anja had already met, but instead he decided to include Lucia.
"WE SAW EACH OTHER TOGETHER FOR MAYBE THREE OR FOUR YEARS." "HER FAMILY WAS NICE, BUT HAD NO MONEY, EVEN FOR A DOWRY."(PG 15)
It is semi-taboo to talk about old girlfriends with your children.  Personally I wouldn't want to know about my parent's sexual partners.  In his account of their relationship, he includes their break up.  Some things he implies while others he makes explicit.  What Vladek did to Lucia can be considered quite cruel, rejecting her partly because of money, and he still chooses to talk about it.  "IT WAS NOT SO EASY, TO GET FREE FROM LUCIA."  The other revealing situation was his war experience.  By the sounds of it, it was pretty traumatic, but he is able to discus it without showing emotion. 
"BUT WHEN I LOOKED IN MY GUN, I SAW A TREE!... AND THE TREE WAS ACTUALLY MOVING. WELL, IF IT MOVED, I HAD TO SHOOT IT HELD UP A HAND TO SHOW IT WAS HURT. TO SURRENDER.  BUT I KEPT SHOOTING AND SHOOTING. UNTIL FINALLY THE TREE STOPPED MOVING. WHO KNOWS! OTHERWISE IT COULD HAVE SHOT ME!"(Pg. 48)
This under-dramatic stoicism feels raw and true to Vladek.  Perhaps, because of what was later to come in the concentration camp enabled (or handicapped?) his lack of empathetic feelings.  In Western culture, a kill like this is despicable; shooting an unarmed person trying to surrender is cold-blooded murder. 
 Art's littered comments through out the book provide us with a lot of insight.  They hint at Art's opinions of his father and even of the book we are reading.  The most powerful tool that Art used to validate this book was the five-page insert of his earlier work talking about the suicide of his mother.  The style of this mini-comic is drastically different from the rest of the book.  The inconsistency it provides, gives it an unedited quality.  Most of Maus is not heavy on emotions with simplistic faces, which rarely reflect feeling: usually two slanted brows to show angry or a tear down a face to show remorse.  Prisoner on the Hell Planet is completely opposite illustrated with dark tones and human faces.  The seriousness of the facial sketches on page 101 hits your hard with the utter devastation Art felt at the death of his mother.   
"I COULD AVOID THE TRUTH NO LONGER-THE DOCTOR'S WORDS CLATTERED INSIDE ME...I FELT CONFUSED;I FELT ANGRY;; I FELT NUMB!...I DIDN'T EXACTLY FEEL LIKE CRYING, BUT FIGURED I SHOULD!" (100)
"BUT, FOR THE MOST PART, I WAS LEFT ALONE WITH MY THOUGHTS... MENOPAUSAL DEPRESSION, HITLER DID IT! MOMMY! BITCH" (101)
Nothing in the rest of Maus sounds like that.  They way Art and his father avoid the topic of their mother's suicide allows us to feel their relationship.  It's easy to empathize with them, but the only problem is if they do not talk about it, the narrator has a harder time of including it in the story; so inserting the mini-comic was an effective way of doing this without tainting by adding too much material.
 The second part of the narrative, the marriage with Mala and the financial issues, gives us a chance to see what Vladek is like outside his own narrative.  Art realizes that his father's story is naturally favored toward Vladek, so he uses the discrepancies as way to bring this to the light.  Since Mala tells Art one thing and Vladek tells him another, we know that someone has to be lying, and probably both are.  Also, the way Art includes these contemporary issues with the writing of the book, in the book gives us more of a general idea and makes me believe it really occurred.  By mentioning the weaknesses of his father's character Art shows the problems with having only one source.  He then hits it home with the passage about his mother's diaries, his only chance at a second source.
"NO. YOU'LL NOT FIND IT. BECAUSE I REMIND MYSELF WHAT HAPPENED...
AFTER ANJA DIED I HAD TO MAKE AN ORDER WITH EVERYTHING... THESE PAPERS HAD TOO MANY MEMORIES. SO I BURNED THEM.
...NO. I LOOKED IN, BUT I DON'T REMEMBER... ONLY I KNOW THAT SHE SAID, 'I WISH MY SON, WHEN HE GROWS UP, HE WILL BE INTERESTED BY THIS.'
-GOD DAMN YOU! YOU-YOU MURDERER! HOW THE HELL COULD YOU DO SUCH A THING!!"(159)
This charged dialogue really punches a lot of the issues together, and it is why it's a good ending (from the perspective of getting someone to buy the second issue).  Since Vladek had not told Art this before, we know that he has at least lied to him about not being able to find it.  This revelation forces us to reconsider all the other information that Vladek provided Art with.  In Prisoner of Hell Planet, Art wonders why his mother kills herself.  I suspect that Art has always in part blamed his father for her death as well as himself and that is why he yells "you murderer."  The second meaning of these words follows the idea that by destroying someone's history, you are killing them.  This is especially painful because his mother wanted her son to read her diaries, which might have relieved Art from some his guilt knowing that it would have fulfilled his mother's desire of him being interested in her.  This missed chance at redemption makes Art understandably furious at his father.
 From his father's semi-shifty accounts Art is able to present us with a believable story by focusing in on the writing of the book instead of directly on the father's accounts.  This makes it different than other Holocaust survivor books; here we can see what's it's like from a different view.  This added dimension of a post holocaust family adds tremendous valuable, partly because it is so personal.  What Art did is opposite of what Hollywood would do.  Hollywood would cut out all the ambiguous confusing elements and polarize the characters to either good or evil.  It would oversimplify the plot devolving it in to clichés that are acceptable to the ideals of the audience.  Spiegelman keeps his story complicated with multi-faceted characters that are not easy to understand, and a plot that mixes truth and lies, good and evil.