It has been said by many that it is impossible to balance the 10 and 25 man versions of a raid due to the expectations of each. If we work under the current assumptions that buffs will not be totally homogenised (Tank, Healer, Magic DPS, Physical DPS) and that abilities will remain scarce (bubbles for damage prevention,2 target healing, shiv, disarm etc). Then it becomes difficult to create an encounter that utilises these rare abilities on a 10 man, however not so in a 25 man where it is reasonable to expect 2 of every class with the 5 remaining slots representing overflow from healing and tanking classes.
Blizzard however actually have a way around this. We saw it in Naxxramas where the 10 man had two mind control orbs and the 25 required at least one priest (after they removed the taunt rotation to stop the boss simply being kited around). This mechanic was seen partially as unfair (making the mind controller act as a tank rather than as a DPS or Healer), however shows the easy way to make fights like this work, the vehicle mechanic / posession,or pets.
So we have a fight that requires a disarming move, on 25 man we expect 2 warriors and 2 rouges, on 1 we might not have either... instead the raid leader pulls out the handy disarmomatic 5000, usable only once (unless you survive) that gives her or her designated player the abilities that are required. It might be an NPC we "call on", a pet (see Grizzly Hills or Zul'Drak), an item we can buy or even one that drops in the fight (see Kael'thas). Working around special mechanics is actually nice and easy, and the added difficulty of a secondary ability or swapping a weapon mid combat is not going to make or break the difference.
The second major issue that we can see are effects that involve movement. Ok things like defile won't fall into this, in that case the penalty should be different such as the tick period or expansion size of the ability to make it equally penalising. However think of Hodir with his NPCs. Except we reverse this, 10 man has 9 NPCs, 25 man has 3 (the required buffs to make the DPS in the fight). Each of these NPCs though acts according to a minimally intelligent script (see the jousting knights in North Icecrown, each appears to have a slightly different response pattern to jousting in terms of stand off and charges). They act as DPS or healers or whatever, but also act to chain abilities. A 25 man now has 25 bodies in it each under intelligent control, the 10 man movement fight has 15-20 only some of them truly intelligent. We can actually balance the fights without having to shrink or expand rooms.
The best bit is that these NPCs can actually be non-friendly such that their job is to cause chaining effects, kiting the adds effectively and positioning them becomes part of the fight (2 people nearby each other representing minimal movement room), they don't even have to do damage themselves, merely be annoying to build the same level of difficulty into the game.