You realize that the overwhelmingly vast majority of tanks in this game don't have the first clue as to how to mark targets, therefore very few dps have ever had any experience with it. They're used to tanks being able to hold a whole group of mobs while they whittle them down. So, are you complaining that you don't know how to keep more than one mob at a time on you, or that no one knows what target marking is? Even tanks that don't know how to mark targets can usually keep a whole group of mobs occupied. So, like, why can't you?