Hi, I don't think they mean anything really. In previous iterations the thought bubbles the sims had seemed to relate a lot more to what the conversation was about. TS4 has got better in this regard since launch but still a lot of them seem very random.
Wait, did you mean the sims thought bubbles or the interaction icons? Umm... just in case this is what you meant...
If you see sunglasses next to a friendly conversation option that is to indicate that using this interaction will level up the Charisma skill slightly. Conversations about skills will all level up that skill slightly so for example a conversation about fitness techniques will give that sim (the one you are controlling and the non-controlled listening sim) a slight fitness skill gain etc.