The worries are many. Will this mean people won't want to do the the 25-mans when they can have the same experience in a 10-man? Mike pointed out in one of our conversations in WoW Insider's secret lair that we were originally told that even 40 level 80s wouldn't be able to take down Arthas. Some are concerned that we really shouldn't be able to take down one of the most powerful entities in Azeroth, and possibly in the Universe, with only 10 people. I think all of these worries are addressable.
What about the Lore?
There's actually been some telegraphing that we'd get some type of change similar to this due to lore reasons for a while. Ever since last year, Blizzard has been talking about making WoTLK more "personal" and making sure people at all levels of the game have a chance to invest themselves in the lore by giving them a chance to interact with major figures such as Arthas. They've already started applying this philosophy in game with the Magister's Terrace, where five people can take down Kael'thas Sunstrider.
In that case, the 10-man/25-man split makes a lot of sense. Letting more people have a crack at Arthas and participating in his downfall makes a huge amount of sense and should make the story a lot more personal for a lot more players. Beyond that, even if you are interacting with Arthas all through the whole leveling process, if you're also shut out from taking the guy down at the end, it might end up being a bit of a buzzkill.
Beyond that, we should realize that the lore power-level of an encounter doesn't always directly match up to the game difficulty rating. After all, as Drysc points out, the Gnomes were chased out of their city by a level 28 that could be soloed by a single Ironforge Guard, and the Trolls by a level 10 and his level 7-9 minions. By a similar token, you can bring a conspiracy that spans half the Eastern Kingdoms and threatens to topple Stormwind to its knees with a group of 5 people of levels 17-20 or so. If all that isn't epic enough for you, 5 level 70s can take down a malevolent force from beyond our reality with the power to destroy worlds with a mere yawn in the Shadow Labyrinth. I could go on all day, but it's pretty obvious that number and level of people required to defeat an encounter doesn't always have an impact on the exact significance of the encounter from a lore or storytelling perspective.
Does this mean it will be too easy?
Another concern of people is that defeating Arthas shouldn't be as easy as jaunting into Karazhan with a bunch of people in assorted dungeon blues and clearing the place in 3 hours. Drysc offers a reality check on that as well. All 10-mans will not be created equal. Even today, there are groups who can clear Karazhan but still struggle with Zul'aman, or at the least have never managed a complete timed clear of the place. Similarly, you shouldn't expect to be able to simply waltz into 10-man Icecrown Glacier, kill Arthas, and go home. Yes, you'll no longer have to coordinate 25 people, but you'll still have to coordinate 10 people, and the dungeons will be constructed with a definite progression in mind. 10-man Icecrown Glacier is sure to be much harder than 10-man Naxxramas, even more so than Zul'aman is tougher than Karazhan. 10-man will still be a challenging progression in its own right.
Does this mean no-one will want to run the 25-man?
Honestly, our own Matthew "The Vykrul" Rossi has probably covered this about as well as I ever could. There's still plenty of incentives for the 25-man raider. There's likely to be bigger challenges from the bosses and more opportunities for more classes. In addition, there's definitely going to be better gear in the 25-mans. This has been confirmed. And since Arena gear will probably still need ratings in Season 5, more people will probably return to raiding as a way to get the best gear -- and I'd bet quite a bit that if Frostmourne is a drop, It won't be coming from the 10-man version of the instance.
Does this mean they went back on their word?
Sure, at one point some CM or Developer said that you'd probably have to have at least 40 level 100 characters to defeat Arthas or something like that, but... things change. Drysc makes a good argument about it -- A Good game company should be adapting and changing. The idea should be to make a game that is fun and accessible to as many people as possible. If this means that you need to make a few changes to earlier decisions or design philosophies, that is something to be celebrated and encouraged, not maligned. In this case, Blizzard is looking to open up the story and encounters of the game in a way that allows a much larger portion of the player base to experience them and have a feeling of investment in the game world. That is a good thing.
I really do applaud this move by Blizzard. It's a gutsy one, but one I ultimately see as being beneficial for the game as a whole, at all levels. They're allowing more people to invest themselves into the lore and story of Northrend, the Scourge, and the game world in general. 25-manners will have more challenging encounters and better loot to look forward to, while 10-manners can progress through the story as well, challenging themselves at their own level. Wrath of the Lich King should evolve World of Warcraft into an even more enjoyable game for everyone, and that is certainly in part thanks to innovations like the new 10 and 25-man split.