Bismillah
CC, let me say something here till Jennifer comes back to the thread, in Islam there is nothing as small, in other words trivial or important. Every thing is important, every little detail has an impact on our lives and relation with Allah one way or the other.
As for how to know what is wrong and what is right, we are commanded to constantly seek knowledge, failure to do so shall not stand as an execuse. The book is there (Quran) the hadeeth are avaliable, we must always refer back to them. Thus, we have a criterion to help us realize what is wrong and what is right.
In case there are differences, for instance on the personal level, I seek those reference to exactly stand on the correct path, then try to correct the other party if he/she is on the wrong, if the person does not listen, then Alhamduelah I fulfilled my role and freed myself befoer Allah.
As for nail clipping, these are sunnah's that we are encouraged to follow. The Sunnah is actually how the Messenger prayer and peace be upon him, did things, how he handled them. Thus, even the nail clipping, I d like to do it the way he did, he must had reasons for doing so, Following it, immitating him, is a source of rewarding, why wouldnt want to do this?
As for whether or not Allah Shall punish one if not to follow, we all seek Allah's Mercy, and I think we did refer to this many times, we cannt decide.
However, Sunnah, is divied into levels, Insh a Allah I will search this and come back to u on it CC.