One of the largest problems in early TBC raiding is DPS players. Quite frankly, most DPS players don't seem to know how to do acceptable amounts of damage.
This wasn't an issue in pre-TBC raiding because the early WoW 1.0 raid DPS requirements were abysmally low. It wasn't until you got to AQ40 that DPS had to start performing anywhere near their full potential.
Let's take Gruul the Dragonkiller. If your DPS does an average of 500 DPS, you are looking at a growth 16 kill. If your DPS does an average of 600 DPS, you're looking at a growth 12 kill. Is asking for 500-600 DPS really too much for your DPS players?
If you look at the WoW Raid & Dungeons forums, you'll see a lot of newer guilds posting WoW Web Stats logs, and asking for help. And time and time again, you'll see the response focusing on the DPS players and how they are underperforming. People not packing enough +hit, using a bad spell rotation, using direct damage instead of DoTs.
If it was just a couple of people with problems, we could say that it was an issue with the player. But the scope of the problem is so large that I think it points to systemic faults in WoW.
DPS players have never had to reach 500 DPS before in the game. All solo mobs and quests are killable with much lower DPS. And it has to be this way, otherwise paladins or other low-DPS characters would never be able to kill anything. So for a DPS player, solo mobs die so fast that there is no need to tune her damage to the higher output.
Boss fights are also much longer than regular fights, and that means that Damage-Over-Time effects become more powerful than Direct Damage. A very common mistake is to see rogues using Eviscerate instead of Rupture. But this makes sense in regular play. Most mobs don't survive long enough for Rupture to finish. Instead, using Eviscerate to kill the mob faster is the way to go. The problem is that the rules change when it comes to boss mobs.
Another common mistake is not packing enough hit rating. There's a large jump in misses between level 72 and 73 mobs, especially for spells. Players who don't know about that jump often don't pack enough hit rating. They go for stats like crit, which have proved more useful in regular play.
The other major problem for DPS players is that they don't have enough feedback. They're just pouring damage into a central pool, and it is really hard to tell if your individual contribution is enough. In contrast, healers get immediate feedback. If a healer doesn't heal competently, people die. This forces healers to improve at a much faster rate. Similarly for tanks, if a mob gets away, or if DPS is consistently pulling off the tank, the tank knows she needs to improve.
So far, the common theme is that regular play, including 5-man instancing, does not prepare DPS players for raid boss fights. And when you combine that with the lack of feedback during boss fights, it's no wonder that DPS players are having problems, especially when first getting into raiding.
The only real way to improve DPS players currently is out-of-game research and theorycrafting. It would be better if there was a more organic, in-game way, to prepare DPS for raiding.