There are several thing I miss from before Cataclysm.
Number one is the number of online friends that I have lost touch with over the years. They moved on to something different in their lives and sometimes I wonder how they are doing.
There are several players that I would like to hear from again. Lon was a player who really got me into running dungeons. He was the ultimate alt guy when leveling alts to max level was really time consuming. At level 60, he had a max level Druid, Mage, Warlock, Hunter, and Priest. I did not get a second max level character until BC.
I miss the late night runs with Melnoch, Sohlam, Shielofyre, Cicela etc. I still keep in touch of all of those players, but they are mostly playing other games at this point.
I miss the way Dungeons felt in Wrath. I found the chain pull aoe fests to be fun. It was not serious business. It was something to do because you enjoyed it and could do a run in 30 minutes.
There are things that you will always "miss" a little bit about games. The one thing you will miss most of all is the people that disappear forever (often without warning).
Thankfully, I have a great group of friends to run stuff with and hang out on vent with, otherwise, WoW could be a desolate place.