Lately there has been a lot of controversy over the way that raiding will be set up in Cataclysm. The way things look now players will have an exclusive lock out that requires them to choose between doing 25 and 10 player instances instead of doing both the way it is now.
To add to this, 10 and 25 player instances will also drop virtually the same loot.

Ghostcrawler (Link)

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I think this is one of those cases where it's not going to be possible to please everyone. Setting aside some of the folks in this thread who want to run both 10s and 25s every week to maximize reward potential, most players either prefer 10s and have no use for the 25s or want to run 25s and don't want to feel like they are being inefficient for doing so. In other words, half (I don't know if it's really half, but it makes the sentence easier to read) the community wants an incentive to run 10s and no incentive to run 25s and half the community wants the opposite. On the other hand, we feel like we need to offer both raid sizes in order to make raiding attractive to a broad swath of the community. As a result, I think it's likely that no matter what we do, proponents of each raid size will feel like we're not being fair enough to their side. It's going to be one of those hybrid vs. pure or PvE vs. PvP ongoing debates that never really get resolved because each party wants virtually the opposite of the other.

This has caused a huge uproar in the community because the upper echelon of raiders doing 25 player content feel as though they're being squeezed out of the game. After all, who is going to go to the trouble of putting together the more logistically challenging and difficult 25 player raids when they can simply grab 10 people and get the same rewards with less competition?

Many players, including non "elitist" types are equally angry that they are being denied the opportunity to run both difficulties of an instance, something that has become common and popular in Wrath of the Lich King.

A huge segment of the players on the forums seem to feel as though they're being forced to play less of the game in order to keep them tethered to some nebulous casual playerbase that will be sad or angry if they aren't able to attain the same gear and achievements as players that play the game more often.

This is a slippery slope and something that I don't believe I've ever heard of in an MMORPG (a genre famous for creating huge timesinks to keep players playing more).

All this stuff is obviously still under development and consideration so weigh in on the forums and have your say. The biggest thread on the topic is here "Why does Blizz want us to raid less?".