I've been doing basically nothing but dungeons since patch 3.3 came out, and the experience was mostly positive. Yes, people still leave in the middle of a long run, but replacing them is a lot easier now, and if it is a dps who left the group is back up and running within minutes. Hint: If the group needs to gather the new arrival at the entrance of the instance, using the eye icon next to your mini-map to first teleport out of dungeon and than back in is the fastest way to get everyone together at the entrance. Yesterday I did BRD several times with my paladin, as tank, and in one case the healer I was originally grouped with stayed with me, while replacing dps several times, until we had completely cleared BRD, and killed every single boss in that huge place. Who would have thought you could clear BRD in a pickup group?
I don't know if lower-level people are just nicer, or whether different servers have different moral standards, but one reader wrote me to tell me he had serious problems with people ninja-looting on the last boss in level 80 heroics. Basically the idea is that the ninja-looter counts on you never seeing him again, so he is safe to roll need to an item he can't even use for his class, and get a few more gold pieces out of the run. Hasn't happened to me yet, so I can't say how frequent that is, but it certainly is possible.
Blizzard *did* think of countermeasures to ninja-looting though: You ignore list now holds up to 50 people, you can set people from other servers on ignore, and the Dungeon Finder won't group you with people you have on ignore. Thus if everyone in the group sets the ninja-looter on ignore, especially the tank and healer, he still might run into problems finding a group in the future. Not a huge threat, but then, ninja-looting doesn't really result in a huge gain.
Did you have problems with ninja-looters in patch 3.3?