Raiding in Cataclysm has been pretty well done for my guild. We beat the first encounter with 9 people in levelling greens and blues and it was a challenge to do so. We continued to beat normal modes with regularity and then eventually moved on to hardmodes - now at 4 of 13. It looks like we are on pace to defeat most, though not necessarily all, hardmodes by the time the next major content patch arrives. This is a pretty good pace to set and I think for semi hardcore guilds like mine Blizzard did a good job providing content. We never hit any brick walls along the way, though admittedly that was due to Blizzard nerfing some overtuned encounters such that they would have appropriate difficulty level. The raids were not perfect when shipped but they did get shuffled into shape just fine for our timeline. The trouble as I see it is with guilds who aren't so good.
Those weaker raiders were very much used to raiding as it was at the end of WotLK. There were many zones that could be horrendously outgeared so people just looking for a raid for fun could go to Naxxramas, Ulduar, Sartharion, Coliseum or Malygos. By the end the 30% buff made even ICC completely trivial and many pugs were doing very well and any guild that wanted to had much of the zone down. This is a pretty easy, forgiving environment for a new or unskilled raider. It was easy to find groups, bosses died without any necessity for really good play or serious preparation and if you couldn't beat the next boss in ICC there was always something to do that felt vaguely productive. This is not true in Cataclysm for two reasons: First, there is only one easy boss. Almost all of Naxxramas was completely trivial and was beating cleared by PUGs within weeks and this was not at all true in Cataclysm. The second issue is that there are no zones that players can easily outgear and smash just for fun. No matter when a weak guild started out in Wrath there were things to do since it shipped with many trivial bosses - in Cataclysm if you can't beat Magmaw or Halfus you just die to them for hours on end until you quit.
Of course the old zones are still there but the ostensible reason for visiting them is gone. The gear in them is irrelevant and they don't provide points so there is no reward for going there. Even though the benefits of running Coliseum in the end days of Wrath were pretty weak people still felt like they were achieving something that forwarded their goal of raiding. There is no plausible deniability any more - Wrath raids are out, to be progressing you need to be killing new content and there isn't much unless you have a reasonable skill level. I think that Blizzard really needed to have something else for new players to do and my theory is that they need an instance with easy bosses that drops worse loot than the current raids. A 4 boss instance with ilvl 353 epic loot would fit the bill nicely - it would allow new guilds to practice on fights that are easy and have a feeling of accomplishment and progression while they wiped to Magmaw or Halfus. Hell, the new instance doesn't have to be new at all, even a Molten Core update would work. The mechanics there are not complicated, if the monsters were updated to 85 it could be a fine 10 boss easy instance for new people to bash through.
People need that sense (whether or not it is actually true) that they are making real, noticeable progress towards their goals. Wiping over and over to the first boss does not accomplish that. It isn't going to change things for my guild and guilds like it aside from perhaps a bigger recruiting pool but it would make the raiding game much more accessible and enjoyable to a big chunk of the WOW population. Whether Blizzard thinks it is worth the development time to produce such raids is another question entirely.