(The following post contains information about Wrath of the Lich King)
Gear consolidation is a new reality being added to the game by Blizzard and I believe it will have a profound impact on guilds both large and small. Essentially, gear consolidation is the attempt by Blizzard to make each piece of gear that drops be appealing to at several classes and specs.
As an example, Protection Paladins, Protection Warriors, and Death Knights are having their mechanics tweaked such that each will want basically the same gear (Plate withStamina, Defense, Dodge, Parry, and Strength)
Healing and Spell Damage are being combined into a single stat called Spell Power, and setting up a conversion of Spell Power to Healing. Now both healers and spell based DPS will want Spell Power gear.
The impact I see this having on guilds and especially smaller guilds is that it may cause these guilds to reevaluate their loot systems for 10 mans.
Most guilds run a straight /roll system for 10 mans. I believe part of the reason for this is that any given piece of loot is sometimes desired, or best used by only one person in the Raid.
If you run Kara with a Paladin/Druid Tank/Offtank setup, and a +DEF piece of plate drops, the rest of the raid is mad, not at the Paladin, but that 'their' loot didn't drop. At most you probably have 2 to 3 people in the raid who even want a given drop. Most players have an understanding of what class/spec needs the loot.
Today, you hear people say "Oh that's a Mage Belt" or "Hmm. Looks like Paladin Plate and a Feral Staff"
Now fast forward to Wrath.
A Spell Power piece of Cloth drops and you have 2 Priest Healers as well as a Warlock and a Mage. All 4 players want the loot for their main spec upgrade. Now, the 3 who didn't get it are mad at the one guy who 'won the roll'.
The situation will be even worse when you start thinking about the non armor specific pieces like Rings, Trinkets, and Necks.
While you will always have generous people in your raid who will pass for others, you will also have people convinced that the loot will never drop again. In a situation where such a high percentage of the raid sees the item as a clear main set upgrade, I don't know how long a /roll system can fly before you start seeing major drama.
I've seen drops cause major drama in guilds both pre BC with items like Zin'Rohk and Dal'Rends, and also in BC with items like Trollbane and Tier 4 tokens.
Gear consolidation may cause even 10 man guilds to need more rigid loot systems like DKP or Suicide Kings (SK) or Effort Points/Reward Points (EPRP).