Typically, from what I've been hearing, pardons traditionally are only given:

a) When the justice system has been allowed to run its course. Otherwise, the executive branch would be telling the judicial branch that it is not trusted to reach the right verdict. Nixon was a special case that was done for the greater good of the country. ie, he was already ruined. The "Party" was ruined. Public trust was ruined and everyone knew what happened. A trial would only have served to shame the country even further. Agree or disagree, but the fact is that was the reasoning. Consider it the exception and a special circumstance.

b) The pardoned must admit guilt and regret for his or her actions. Otherwise, you are pardoning someone for an act they deny taking part in. Clinton is generally thought to have ignored this one, but again, it's considered an exception rather than a rule.