Haven't really ever used the Simple view, but did find an explaination for you (P6 manaul p.229 or P7 manual p.196) :-

Quote - A Caution symbol (exclamation point in a triangle) will appear at any point when the
surface attributes you are working with are too complex to be edited in the Simple tab. For
example, this would be the case if a procedural shader node were attached to the diffuse
color input. Whenever the Caution symbol appears, it is an indication that you need to use
the Advanced tab for full access to this attribute’s configuration parameters. Click the
Caution symbol to switch to the Advanced tab.