LFD is there so when my 4-man guild group is short a DPS, we can fill the slot with some random shmoe. If said random shmoe does a modicum of damage without breaking CC, random shmoe gets an easy shot at heroic loot. If random shmoe is nice but incompetent, we’ll try to teach him how it’s done. If random shmoe is stubbornly bad or vile/homophobic/irritating, we tell him so and boot him back to the 20-minute queue.
I’ll LFD in 3-man guild groups too, if we have a guild tank. It’s not worth my time to try pot luck on tanks, or to enter a heroic without a vote-kick majority.
LFD is a great tool for finding mediocre players, so it’s great for running nonstop easy content (like the LK heroics back in the day, or 85 normals now) or topping off a strong group for hard content. But it’s unreasonable to expect all 5-man content to be so easy that you can do it with nearly any combination of strangers within a couple of months of the expansion’s release.
By the way, chalk this Holy Priest up in the column of those who enjoys Cataclysm PVE healing about as much as vanilla, BC, or LK healing. I’ve got a pretty full toolbox to choose from in any encounter--Lightwell and Chakras and Mass Dispel and Psychic Scream and Leap of Faith and Divine Hymn and more. Watching for the opening to apply these tools keeps me from getting bored.