I think the icon thingys are envelopes..?

I'm guessing it's triggered by the amount of replies you get. Currently before I reply the icon looks like this on the thread ---

<--- A closed envelope. = No replies...?

---------------------------------

<----- Under 3 replies...?
<------ Last poster was OP....?
<------ More then 8 replies?
<----- Lots of replies...?
<---- Lots of replies/ views..?
<------ Locked?