General state of raids and instances
As you may know (or not), instances and raids are artificially buffed on this realm.
Instances have roughly 33-37% hp and melee dmg buff. It applies to all trash and bosses and I think it affects also dmg effects (poisons, bleeds, explosions etc.). As far as I observed, they give a rather rough challenge but not huge enough to be considered unbeatable. Mostly it just lengthens the fight by having to do 33% more damage to boss before he dies, on some rare cases you gonna have to pop some defensive cd but otherwise it looks OK.
Raids on the other hand have much higher buffs. It varies between ~50% (most 10-mans) and 100% (meaning boss has twice as much hp and melees twice as hard) on 25-mans.
I'm okay with that, we know there should be a challenge but I think the values may be overshot a bit, especially for now-a-bit-obsolete raids that do not give a precious loot for hardcore raiders such as 10-man Naxx and VoA.
Those raids should act as a entry step towards raiding to create a healthy progression. However some of the bosses are undoable with pugs which is not really fun if you are going to gear yourself. For example. 25-man Archavon has 25% more hp than Toravon did on retail, which was a boss on retail being done with groups that did twice as much dps (rough estimate on dps chart on groups with ~5400 gs).
Challenge? Well yes, but I think there should be a healthy limit. Many pug groups are being disbanded if they consist entirely on new players, because 200 blue/epic geared characters are not able e.g to beat enrage timer of 4 minutes with such a overpowered (in terms of the health pool, not exactly damage done) boss called Archavon. You may say "haha bring some better players n00b" but that's not the point of raids that not a part of current progression. If you are not able to beat a rather easy boss without having 2/3 of your raid with overpowered gear then something is not okay. Naxx and VoA were supposed to be a step higher than heroics (and this fact is reflected in loot) but here they appear to be two steps higher, which is a bit unfair given the loot that they give.
There are two options to resolve that:
1. shift loot tables tier or half a tier up to make them more appealing (you work much harder than you should to beat a boss, but at least you have a chance to receive a reward that is useful to your main spec) - that's not really what I would see as this would create confusion among new players and could create issues with loot
OR
2. consider rebalancing raids in terms of HP and dmg added. People are already clearing Ulduar 25 HMs so if you are OK with that then keep the buffs on current tier but rebalance other raids so the progression is much smoother. 100% flat HP and dmg increase for each boss (more or less, values are not equal for all of them I think) could be smoothed out between 40% (Naxx entry quarters) going through 50% (Sapp, Kel) and up to 60-70% for Sarthartion and Malygos.
As I said, overall the idea with the buff is okay but the values itself seem to cross the gap between "fun new challenge on 12-year old content" towards "being frustrating to do even on moderately-geared characters with no useful loot whatsover".
Blizzard spends a lot of time tuning the encounters and there is a reason for that. They have much more tools in their arsenal (changing the number of phases, their length, number of adds, abilities timers etc.) and we have to stick to plain HP/dmg changes, but there should be a sane limit to that. Doing 12 dances on Heigan instead of retail-like 2-3 will not make the encounter any more fun than it was and this applies to every other boss as well. Even if you master the dance, a boss still could kill you with unlucky streak of disease cast and melee hit which is something not seen on retail as Heigan's melee damage was very low. You can obviously mitigate that with a character that has 40k hitpoints, but then his loot will not be interesting anymore.
TL; DR
There is too much gap between heroics and entry-level raids and this gap is so huge that it doesn't add any fun-factor to the game. It's okay to tune encounters up to add some spiciness (with the prime example of heroic dungeons), but for many bosses instead of adding a pinch of salt an entire wagon of pepper was thrown.