Ixobelle is challenging us to design a fun raid encounter, claiming that complaining about such an encounter is a lot easier than designing a better one. True that, but *why* is fun so hard to design? Because most people think a particular encounter is fun when they are able to beat it, but it isn't a pushover. And it is very hard to predict for who that will be true.
The big problem is that a character being at the level cap doesn't tell you enough about him. If I take a pickup raid of freshly minted level 80 characters in green / blue gear with no experience of working together, and compare that to some uber guild raid group in full heroic epics, voice chat, and countless hours of practice, the difference is enormous. The top end raid group can easily withstand multiple times the damage, and simultaneously dish out multiple times the damage per second, than the pickup raid group. Even inside one guild there can be huge differences. Friday I was on a guild raid which easily killed Sartharion, and cleared the plague and military wings of Naxxramas, except for the 4 horsemen. Sunday I was on a guild raid with the same guild, but with several raid members new to Naxxramas, and we barely managed to clear the spider wing, which is supposedly the easiest one. But in some respects the less successful raid was more fun, as we really had to work on our strategies.
With gear, experience, and communication making such a big difference, it is nearly impossible to design a raid encounter which is challenging, and thus fun, for everyone. When we recently visited Molten Core with around 20 people of level 80, the place was such a pushover that there was no fun to be had beyond nostalgia. An encounter like Baron Geddon is fun only if YOU'RE THE BOMB actually matters; if you can explode in the middle of the raid and nobody notices, the boss ability is just a waste. Ixobelle talks about the Gruul encounter, but that again was fun only if your raid was in a certain range of damage dealing and damage absorption capacity. If you were so badly equipped that you'd lose even if you played perfectly, or if you were so well equipped that you'd win even if you played badly, then there was no fun. At level 80 all of the classis WoW and TBC raid dungeons are too easy to be fun, but if they wanted Blizzard could simply increase the health and damage output of the trash and bosses in all of these places, upgrade the loot table, and have fun raid dungeons again. Maybe some encounters (Razorgore in BWL for example) would be hard to tune for 10-man raids, but other than that there is no reason why a recycled Molten Core couldn't work exactly as well as the recycled Naxxramas does.
So my entry for Ixobelle's raid encounter design challenge is easy: Just tune Molten Core to be a viable 10-man or 25-man raid dungeon. Which means roughly multiplying each mob's health by a factor of 10 (Lucifron now has 350,000 health, a typical 10-man raid boss in Naxxramas has around 10 times that), increasing the damage output accordingly, and filling the loot table with level 80 epics and emblems of heroism / valor. Or maybe another classic WoW dungeon, there are still a lot of people who have at least seen parts of Molten Core, but never been to places like BWL or AQ40, which is exactly why Blizzard recycled Naxxramas. I think most of the raid bosses Blizzard ever designed have the potential to be fun, they just weren't always tuned to a good difficulty level.