I can't really say what it is, but I just don't get the thrill most people do when obtaining an achievement. When we did a couple of achivements as a guild group on one occassion, I enjoyed playing with other guildies for a common goal, but when it dings that I have obtained an achievement I'm kinda "meh" about it.
I know I have guildies that are working very hard to get achievements done but I just don't have the enthusiasm for it. That being said, I get my rush from other ways in the game that perhaps are "meh" to you. When I strike an incredible deal in the AH, when I have a full page of AH sales, when I kill a difficult mob or perhaps escape an enemy.
I absolutely respect those who are into achievements and I know some of them are crazy difficult. One of our guildies, Miss Millie, is working on the Mountain o' Mounts achievement. Now that is some freakin' achivement to get. Perhaps because there are so many of them, they don't seem like achievements. I'm sure I /farted in an instance the other day and got an achievement hehehe. Sometimes it just feels that way.
Maybe if there were achivements in the area of making gold or sales, I would feel differently. I'm kinda surprised there aren't achivements in AH sales. Making 5k in a day or selling 100 items etc. Those might be achievements I'd be interested in and I think there would be a large number of other folks that would enjoy them.