100% perfect Balance:
This is not attainable, not for all bosses in all raids. There will be no perfect, 100%, absolutely equal difficulty between a ten-man boss and a 25-man boss, not without years of testing and balancing and perfecting, and even then, it's not likely, not for all the many bosses and encounters that require attention. While theoretically attainable (anything is possible, after all), it is unlikely to be practical, at least in terms of a development team's time commitment. This is not my idea of balance.
A 5% difference in either direction allows for some wiggle room for raid size to impact an encounter's difficulty: insta-wipe mechanics, room and need to spread, aoe size (both damage and heals), number of adds, available class-specific mechanics, etc. This can be brought closer to 98/99% by providing a mix of mechanics in the instance that reward and punish the different raid sizes: a 10 man may wipe a lot on Boss A and push Boss B over quickly, while a 25-man may spend more time than the 10 on Boss B, instead.
Given practical time commitment, it is likely that patches would be required to more closely fix a misbalanced mechanic that slipped through testing. It is unlikely that a Sarth3D difficulty difference would be left for long, for example... but a situation like the Rotface plague timer (set even to 25-man on its 10man release) may still happen and be hastily patched again as it was in WotLK.
Perception of this balance may be skewed by Blizzard's wish to more quickly gear up 25s, depending on whether or not the upgrade-speed is factored into their planned balance. Guilds are unlikely to know, going into an instance, that the other raid size is x% easier than their own raid size, because the balance will tip slightly in each encounter towards one side or the other. The overall percentage tally for the instance will be mitigated and altered by things Blizzard can't or won't control and we can't easily numerize: player skill, disconnects, mod use, user graphics hardware, guild morale, emergencies, etc.
I suppose the best way to describe it is with the saying, "close enough for government work."
This is my idea of balance: not extreme, but viable.
Some people don't like the idea of 10s and 25s being equal, and refuse to beleive they should be balanced. Many times, these are players who have no perception that their gear is actually making the WotLK ten-mans easier for them, and are used to traipsing through a ten-man for giggles and badges while using their 25-man gear to more easily survive the mechanics that are otherwise brutal to those with less health, dodge/block/parry, attack/spellpower, haste/crit, and mana. Their idea of a raid balance is to have an "easy ten for lawls" and a "true progression 25." They want this. They think that is balanced. They're in for a rude awakening, based on what the Blizz staff have told us.
So this brings me to wonder on the unanswered question:
How is Blizzard going to judge the balance?
- Are they going to balance against gearing speed in 25s? Can we expect 25s and pure10s to actually progress at the same speed, based on gearing, or will we still have ten-strict rankings for a separate progression list?
- Are they going to balance based on relative ratio of healers and tanks per party?
- Will it just be based on how quickly two similarly skilled teams of players tackle the instances?
- Will the balance be tallied per boss, or per wing, or per instance?