there aren't really many unplayed factions in the DL group. even mercs and minions show up sometimes. there are some that don't see quite as much play, mostly ones that one or two guys have as a secondary faction. Recently it has seemed like Circle, Menoth and Kahdor have been lacking. I'm always trying to get more people to play Skorne, I'm usually the only one with them at the store. though last week there was some more skorne on the table and I have seen a few people trying them out.
on the other side, Ret seems to be everywhere. if you like the faction don't let that stop you from playing them, though. but based solely on how many ret players/armies we get on a given week, they are probably the most played faction (strangely enough)