It's pretty individual. I had mild tremors, nausea, sweating mainly. Mental distraction, inability to concentrate. Also sleep disturbances and extreme tiredness during the day.
As miserable as heroin withdrawal can be, alcohol withdrawal is far more dangerous. An alcoholic who is withdrawing can die as a result of seizures or heart failure. That almost never happens with other drugs. And there is no way of predicting who will have extremely serious withdrawals--which is why medically supervised detox is always recommended.