The premise is simple: Post about your favorite details, painted or molded, or maybe ask if they have significance. I'll start off with a few:
Firstly, something semi-humorous from ROTF Tailwhip, his TM'd Decepticon symbols:
Next(since it's an obvious one), Classics Powerglide's Heart(referencing "The Girl Who Loved Powerglide", naturally):

The face of the Ninja![Turtle] As seen on Energon Insecticon.

I recall I asked why it was there once and the reply was "the designer just wanted to give a nod to TMNT". Just a nifty detail I hope this thread can uncover similar instances of.
Now for the last one, I do wonder if there's any significance. It comes from Cybertron Optimus Primal, underneath his chestplate:
Here's a simple paint rendition since the pic isn't the clearest:
Any idea if it means anything? I think it looks like the Key to Vector Sigma. I would mark it off as just another detail, except it's the ONLY detail on that particular piece of plastic. Always struck me as being curious.