This only took 6 years to implement. It indicates if the tracked item (this works for all indicators shown on the mini-map) is at a higher or lower altitude than you.

Simply a vertical indicator. If the node is about the same level as the character, the triangle goes away. If the node is lower, it changes to a down arrow.

But I also didn't check with nodes that were anything other than gray -- flew around Eastern Kingdoms a bit yesterday, my dwarf's mining is 400+ (don't recall exact number off the top of my head). I'll check in a few minutes to see if nodes that could give skill points have different colors for the indicator.

Apologies for the absolutely awful JPG compression. I circled the node to help show it off, and cropped a bit then saved using MS Paint.

Also shown is the archaeology dig site indicator.
Also also shown is the transmogrification of a few pieces of armor, using whatever I had lying around with my character copy. Not final and I wasn't going for a decent look, just messing around.