Wormtail, while not the most faithful servant, was the person responsible for Lord Voldemort's resurrection, and possessor of a very cool hand. He was the one who allowed Harry's parents to be found and killed. Why then, after all that he did for Voldemort, was he cast as a mere servant?
Throughout Harry Potter and the Goblet of Fire, Voldemort drops information as to why Wormtail was not given a higher position in the Death Eater circle.
Foremost, Voldemort doubted Wormtail's loyalty. In chapter one, The Riddle House, Voldemort castigates Wormtail, telling him that Wormtail's devotion comes from cowardice, not from true loyalty.
"Your devotion is nothing more than cowardice. You would not be here if you had anywhere else to go." Voldemort later states, "How am I to survive without you, when I need feeding every few hours? Who is to milk Nagini?"
Goblet of Fire
So, Voldemort is physically dependent upon Wormtail, thus he must retain him as a lowly servant. However, he refers to Wormtail's care as "clumsy". He goes on to tell Wormtail that if Wormtail follows Voldemort's plan (to get Harry Potter for the purpose of making the potion that regenerates Voldemort's body at the end of Goblet of Fire) Voldemort's "faithful" servant (Barty Crouch Jr.) will rejoin him and Wormtail. When Wormtail asserts that he, too, is a true and faithful servant, Voldemort says:
"Wormtail, I need somebody with brains, somebody whose loyalty has never wavered, and you, unfortunately, fulfill neither requirement."
Goblet of Fire
Voldemort does not consider Wormtail intelligent or loyal. As well, I would postulate that Voldemort considers Wormtail a liability, due to Wormtail's life debt to Harry Potter following the events in Harry Potter and the Prisoner of Azkaban.
Regarding the silver hand, Voldemort gave the hand to Wormtail; Wormtail didn't come to Voldemort already possessing the magical hand. As well, as we find out in Harry Potter and the Deathly Hallows, the silver hand is both friend and foe. It serves Wormtail as a functional limb, but when Wormtail hesitates to kill Harry (Deathly Hallows -- chapter twenty-three -- Malfoy Manor), the hand turns on Wormtail and Wormtail strangles himself. Voldemort had enchanted the silver hand to kill Wormtail if Wormtail showed weakness in the form of mercy. Therefore, the silver hand cannot be considered an advantage in a servant.
Yes, Wormtail was the one who revealed the location of James and Lily Potter to Voldemort. However, Voldemort failed to kill Harry, and was almost killed himself by his own rebounding curse. Thus Wormtail's information did not result in success for Voldemort. Harry Potter remained alive and, as we learn, all of Voldemort's further attempts to kill Harry are thwarted. Wormtail's information is ultimately useless.
Voldemort granted only his most loyal Death Eaters a place in his inner circle -- those Death Eaters who endured Azkaban out of loyalty to him. That said, all of Voldemort's Death Eaters were in fear of him, and knew they were all at risk of being punished by Voldemort for even the slightest misstep:
"If we are mistaken, if we call the Dark Lord here for nothing . . . Remember what he did to Rowle and Dolohov?"
To Voldemort, none of his Death Eaters were indispensable; he disdained everyone but himself. He did not grant true power to any of his Death Eaters, to operate autonomously or under their own volition on his behalf. Voldemort's contempt for his Death Eaters was also demonstrated when Voldemort summons the Death Eaters to the graveyard in Goblet of Fire, following his resurrection. He casts the Cruciatus Curse on the Death Eater Avery when Avery begs for Voldemort's forgiveness. He goes on to say both to Wormtail and in general:
"I do not forgive. I do not forget. Thirteen long years . . . I want thirteen years' repayment before I forgive you. Wormtail here has paid some of his debt already, have you not, Wormtail?"
He looked down at Wormtail, who continued to sob. "Yes, Master," moaned Wormtail, "please, Master . . . please . . . "
"Yet you helped return me to my body," said Voldemort coolly, watching Wormtail sob on the ground. "Worthless and traitorous as you are, you helped me . . . and Lord Voldemort rewards his helpers . . . "
Goblet of Fire
Voldemort then proceeds to conjure Wormtail's magical silver hand. But he never considers Wormtail worthy of being a right-hand man.
I think it boils down to the fact that he just wasn't a particularly powerful wizard. Voldemort doesn't care about anything but power. I get the idea, though it's not explicitly stated, that all his other higher-ups are fairly powerful. And I'd bet it doesn't help his position that he's in debt to Harry for his life.
Because he is not loyal to Voldemort
Pettigrew certainly is a Death Eater. He has the Dark Mark, which Voldemort used to summon the other Death Eaters in Harry Potter and the Goblet of Fire.
As to why he is treated poorly, it is because he is low in Voldemort's regard. Many of the other Death Eaters serve at least partly out of loyalty to Lord Voldemort. Others, such as Lucius Malfoy, have no great loyalty to him, but support his goal of pureblood supremacy. Peter Pettigrew is different:
“You returned to me, not out of loyalty, but out of fear of your old friends. You deserve this pain, Wormtail. You know that, don’t you?”
—Harry Potter and the Goblet of Fire
Voldemort has contempt for Wormtail, since he has no loyalty to Voldemort or to his cause.
Because he is weak
Generally speaking, Voldemort has nothing but contempt for weakness:
“You mean he was weak!” screamed Voldemort. “Too weak to dare, too weak to take what might have been his, what will be mine!”
—Harry Potter and the Deathly Hallows
Many of his Death Eaters would have the same attitude. In addition, they push Wormtail around becase they can. He is not prodigiously talented in magic, and, what is more, lacks the courage to stand up to the other Death Eaters.