Details, just because IAAC asked for them.
The ones I heard of (and I never served on one) were filed mostly for the reason that the person felt the decision of the committee to disfellowship was wrong. The person felt they should have been found "repentant." In a couple of those cases, an appeal committee agreed and overturned the DF. In a couple of other cases, the appeal committee actually found that the evidence was lacking but upheld the DF anyway, stating that the person had an unrepentant attitude about the help offered them concerning some minor sins still involved.
I know that's confusing. A person said "I am not guilty of what they DF'ed me for." An appeal committee investigated in a new JC and discovered that guilt could not be established according to the rules. But the appeal committee upheld the DF anyway, stating unrepentance for some other violation of the rules. Basically, it would be like overturning the fornication portion of the charges and then adding lying to the elders in to uphold the original decision.
For the most part, appeals simply upheld the original DF's. But the rules are so strange and so many elders don't understand them correctly concerning what is and what is not proper evidence. They don't often understand that lack of cooperation on the part of the accused is not any evidence at all. Appealing any decision buys you time before an announcement can be made.
There is no reason not to appeal any JC, unless you want to be reinstated sometime soon. I tried to tell my best friend at the time to appeal his DF decision when he came forward and confessed to a violation of the rules concerning viewing pornography. He told me that he wanted to be reinstated quickly, so he would not appeal. His actual DF, when he filled in the details, were for a single word answer to the elders- they asked how his "problem" was going. He said "Fine" knowing they were refering to a previous confession of viewing pornography. Later, he approached them and said, "I said it was fine, but it's still a problem. I need help." Because he was an MS, they removed him, but decided to have a JC for lying. I thought it was ridiculous, but that's what they did.
He rationalized that he did indeed lie (with that single word, "Fine"). He thought if he lost the appeal, they would keep him "out" longer. That's probably true. The same elders that you appeal against have to recommend your reinstatement.
Most people learn after going through any JC to stop telling the elders anything.