It is pretty safe to say you will encounter a lot of people crammed up next to one another having too much alcohol. Is that good or bad? Depending on what you really are looking for it could be the time of your life or the longest night you have ever spent.
My suggestion: If you really enjoy the large, loud, parties with excitement that borders chaos, you should thrive on the Las Vegas Strip on New Years Eve. Expect plenty of situations with hundreds of strangers and an enormous amount of hugging and kissing.
If you like it a little more controlled get into a Las Vegas club instead.