Complaints about patient safety and the care of dying patients dominate those referred on to England's NHS watchdog.
Concerns over hospital care made up just over half of the complaints seen
The Healthcare Commission - which is to crack down on NHS trusts which handle complaints badly - analysed 16,000 complaints received since July 2004.
Cases included a child getting the wrong injections and a bereaved family being shown the wrong body.
NHS trusts said the fact 90% of complaints were handled locally showed good systems were in place.
The commission will audit 50 trusts, including those found to have failed to resolve enough complaints at local level.
The worst trusts could see their annual health check rating fall. A weak rating means a trust cannot bid for foundation trust status.
The best performers will also be analysed to see what they are doing well.
About 100,000 complaints are made annually about NHS care.
Before the Healthcare Commission took on responsibility for handling those complaints which are not resolved at local level, about 3,000 a year went to local independent conciliation.
The commission now deals with about 8,000 unresolved complaints a year.
Just under 70% of those complaints within the commission's jurisdiction were upheld in favour of the patient.
The commission found that just under a quarter of all the complaints it dealt with involved safety concerns, such as a failure to properly manage problems after childbirth.
Half of complaints about hospital care related to the care of a patient who was dying. Families complained about receiving contradictory information about their relative's condition and about having too little time to prepare for a loved one's death.
There was also one complaint by a family who were given a relative's possessions in a black bin-bag.
In relation to GP care, the biggest concern was around misdiagnosis or delays in referring patients, accounting for 66% of complaints.
Healthcare Commission chief executive Anna Walker said: "Complaints represent the raw feelings of patients and the NHS must listen and learn from them.
"At the centre of each one is an individual who has genuinely suffered.
"Too often, this was not just because of what went wrong but because of the way people were dealt with."
Marcia Fry, who oversees complaint investigations at the commission, said: "This is about basic measures. It's not about more money.
"We are perplexed as to why they have not already been implemented."
However, Lancashire Teaching Hospitals NHS Trust - one of those named as a poor performer - "categorically refuted" the commission's figures.
A spokesman said: "The Healthcare Commission has inaccurately tried to portray us in a bad light at resolving such issues locally."
However, patient groups warned the level of complaints to the commission vastly underestimated the real level of concern.
A spokeswoman for the Patients Association said: "The number of patients who are dissatisfied with the way in which complaints are currently handled, will be far higher than those who choose to make a formal complaint; the process is cumbersome and time-consuming.
"Many patients and their families become frustrated and simply give up."
And Gordon Lishman, director general of Age Concern added: "The nature of the complaints received shows a distressing picture of safety and dignity of patients being ignored.
But Gill Morgan, chief executive of the NHS Confederation which represents the majority of NHS trusts, said: "Over 90% of complaints are handled at a local level, so most trusts have good systems in place to respond to patients and their families quickly and appropriately."
Health Minister, Lord Hunt said: "Dealing with complaints in a quick and effective manner is central to top quality patient care and is something that every NHS organisation should take seriously."
He said the Healthcare Commission would continue to work to tackle a backlog of complaints and the department would continue to monitor its performance.