Warning: contains nerd raging and old content. I will try to avoid swearing.
Naxx is clearly too easy. How can I tell? Take someone that has cleared Naxx and stick them in an old world raid and they will fail completely. I'd say they'd fail over and over, but they'd have left after one wipe. This has happened over and over. I'll go over some common failures.
Core hound packs must die within a few seconds of each other. Apparently this is too complex for DPS to understand. They pick a dog and stay on it. And stay on it. And as dogs res each other and I call for them to stop DPS, they stay on it. Eventually I started just soloing the packs.
Shazzrah: Is purge/dispel/shield slam really that hard? He can still hurt if you're letting him deal 50 or 100% more damage, I forgot which. The point is, it's a simple task and still people fail horribly.
Gehennas, Magmadar: Don't stand in the fire. Is that too complex?
Don't stand in front of the dragons that do Shadowflame. I will tell you which ones. Don't stand in front of them. IT IS NOT COMPLEX. Also heal me rather than running around the corner because you're afraid of flame buffet. Yes, this happened and was the last straw to trigger this post, I kept dying because the healers were around the corner and the elemental shaman was too busy throwing lightning bolts. If I detect fail like that again I'm turning him towards the raid and bubble-hearthing.
Suppression Room: Keep moving. No really, keep moving. I know it's slow, but keep moving. Move. Move, move, move. I don't care that a whelp is biting you, keep moving so you don't fall behind. Now put your back to a wall so you don't get knocked off the edge.
Temple of Ahn'qiraj
Don't kill the MCed person.
Follow me so you stop getting lost. How hard is it to follow someone? Every time, someone gets lost. I don't know how. I mean, is it that complex to follow someone?
Interrupt the heal. Yes I know you're probably all the same noobs that want everyone in the game to have MS just because you're too terrible to interrupt heals, but for the love of god, INTERRUPT THE HEALS!
Stop complaining about mobs with whirlwind. Just don't stand next to them. If they attack you, run to the tank. Tanking classes, use your damn taunts. Yes, DKs, I know you're blood, but first of all, blood can tank, second of all, it's a level 60 mob and you wear plate, so taunt. Idiots.
Stop asking "why do you keep needing X?" Do you know what it is for? No? Then WTF do you care? I'll humor you. These are part of an old system of gear distribution. They are useless unless you have one of the tokens. If you get a token and don't drop raid the second C'thun dies, I just might give you all the little bits you need. Of course they're still useless because none of you have the slightest bit of rep because you just want to get run through once for the achievement and in the process get full 2.5, except you don't know what 2.5 is. And on that note, STOP NEEDING ON EVERYTHING THAT DROPS WHEN HALF THE TIME YOU CAN'T EVEN USE IT.
Twin Emperors: Stop asking why they are healing. Stop telling me to just pull. Stop AoEing the bugs. Stop using DoTs. Stop failing at listening. You know why I am taking so much time to explain the fight? Because it's almost complex. In fact, if not for the level difference, even ignoring gear requirements, it's harder than any fight in Naxx. Due to our level, it's not as complex as it used to be, but somehow people still fail.
Here's the idea: I tank one one side, another paladin tanks on the other. They go back and forth and they die. Heal us, kill bugs if they get mean, don't AoE, don't use DoTs because they screw up aggro. Don't lie to me as say you didn't use rupture when I can not only see it on the mob, but right after the teleport in runs straight towards you.
C'thun: Don't all run in at once after I say to wait. Pick up the damn cues and notice that no one else is inside the room, so don't go in. Hunters, don't do retarded crap with feign death. Kill the tentacles, kill him, don't stand in front of or behind him during glare, spread out. Yes don't stand behind, it's a bug, deal with it.
Angry Paladin is Angry
Why are people unable to do content 20 levels old? By now the fights are much simpler and half of them can be facerolled. But any fight which requires anything more than facerolling, people fail completely. Then they all leave. I think this is because of Naxx. It's too easy. People get mad if a boss takes more than one attempt. Should Naxx be hard? Not exactly. It is an intro raid after all. But as it is currently, it's far too easy. Maybe 20% harder would work? Whatever it is, a boss should not be a one-shot on the first attempt unless it's intentionally the one easy boss per raid. I think 2-3 attempts is a decent target for an intro raid. It's enough to challenge but not so much that it's an awful time sink with 100g+ repair bills and massive consumable requirements and all that.
Make Classic Raider a requirement to enter Ulduar. Make people do a challenging fight for once, especially a challenging fight that isn't just there to hand out loot. I did Kael'thas with my guild Wednesday night. It was a lot of fun. We died a couple times while learning it, but that just makes it more fun. I was happy when I died and when we wiped. Sure I wouldn't have wanted to wipe for three hours, but a couple wipes make the end result more worth it.