Colin
As I understand it, LR uses the jpeg preview attached to the picture file when it first shows the thumbnails on screen. It then goes on and produces higher resolution previews from the RAW files ( I am assuming you are shooting in RAW). Are you shooting in auto white balance, or have you selected a particular setting? The odd thing is the fact that the previews are ok, but then they look greatly over exposed. It is not a problem I have ever had with the numerous cameras I use, albeit none of them Nikon.
Although I am sure another forum member might come along soon and answer your problem, in the meantime it is not necessary to individually adjust each picture in turn. If all the pictures in a batch are say over exposed, just adjust the first one, then select the whole batch and hit the Synchronise button at the bottom right of the screen (in Develop module). You can then select which parameters you wish to adjust on all the other pictures to match the first one.
I should also say that though I have been using LR since it's launch nearly two years ago, I am still learning new ways of using it every week. You might also just need to read a bit about it a bit, and believe me I have spent many hours learning the programme!
Good luck
Jim