In the last week or so, I have taken a few calls asking. "Why can't I rotate my model a full 360 degrees"?
The solution is quite simple.
Your 'Rotate About The Scene Floor' option is checked. It's actually a cool feature.
When this is checked, it locks the vertical axis in relation to the front view during view rotation. This prevents the model from tipping and tilting about the horizontal plane and creates the impression that the model is on the floor. Your feedback cursor will look like this when rotating.
Let's go ahead and turn this option off.
Right click in the graphics area and half way down you will see 'Rotate About The Scene Floor'.
There you go, 360 degrees of freedom.