I thought of this couple times, let me explain what it is and why would it be nice to have such a feature. (imo)
Once you finish a random dungeon with strangers, grouped via LFD, a little window pops with their nicknames, and you can rank them 1 to 5.
You could rank whatever you like, maybe that players' performance, maybe attitude, anything.
If you rate him 1 - you will never (or extremely rarely) get grouped with him again (which is kinda already happening, you rarely see someone from another realm that you did a dungeon couple days ago).
If you give him 5 - game automatically tries to group you with that person the next time you queue fort LFD (assuming you're not in that person's "1 list").
Basically the lower number you give a player, the less chances you have to be grouped with him again.
What does it solve?
Well sometimes I get grouped with such players that i'd slap them xD And my ignore list isn't big enough to ignore everyone I really really disliked playing with.
While other times I like some players really much, either I like their performance, or that they are nice guys/gals, etc., but I don't feel to know them SO good after one dungeon that I'd put them to my real ID friends list so I could group with them again.
Oh and inb4 someones says that would increase wait time by gazillions hours, I'd say the game should be looking for players from your list of "5s" for 10 sec, then 10 sec for "4s" etc. etc. and ~30 sec later would just queue you the normal way if nobody from you "preferred" list is available.