One thing that beginners can't spot are what are called pumps. They really are only 100% visible when riding along the track in the 3D editor, but an experienced person can look at a particular section and just tell you it's going to have a pump.
The only way to rid the track of these pumps is to practice practice practice! It may take a couple years before someone gets good enough to get 7-8 out of 10 on a rating of the track smoothness. I've been using NL since the earlier days of V1.5, and I tell you, the built in auto smoother does make things a lot easier than having to smooth by hand.
However, even the auto smoother can't fix a pumpy section on its own. You need to be able to build the track so that the auto smoother will smooth it properly. And the more segments you use, the more control over the element you have. For a loop, don't use 3-4 segments, use something like 8 segments, and you'll have plenty of control [;)]