People talk a lot on the forums about distinctions between casual and hardcore players. But many non-raiders play several hours daily, while many progressed raiders only play a few hours per week. If you're getting your valor cap from RDF groups every week, you're more hardcore than many raiders, if hardness is measured by time played.
The real split is between those who have an organized, regular guild or raid group and those who do not.
I've been doing random dungeon finder groups on my warrior-tank alt, and what I've learned is that random players will almost always fail any execution check. If there's a predictable effect or attack, somebody always stands in it. If there's a mechanic that requires movement or a target switch, somebody always ignores it. This is often true even if you explain the fight beforehand.
Random groups only become smooth when the players are powerful enough to just muscle through the fight without responding to the mechanics. So, even after 4.2, when most people will have access to 359+ gear for half their slots, and the 15% buff on top of that, dungeon finder groups will continue to wipe on fights like Ozruk and Venoxis.
Organized groups and guilds aren't necessarily composed of people who are faster with their fingers or smarter than pickup groups. But by playing together, they learn the fights together. Then, when they return to them the next week and the week after, they already know what to do. The boss tends to die quickly and the guild moves on to learn the next fight.
In pickup groups, the knowledge gained through repeated attempts dissipates. You will wipe five times today while a tank figures out how to position Venoxis so he can run through the boss when he channels the breath attack, and the next time you run the dungeon, you'll die five more times while a new tank learns it.
Heroic dungeon fights are relatively simple. Bosses have a small number of abilities to learn, and there are few phase-changes that fundamentally alter what's happening in the fight. Approaching these fights in the minimum appropriate gear with no knowledge, most groups should be able to figure them out in less than an hour.
Raid fights, by contrast, usually have several things going on that everyone in the raid needs to learn about and understand how to handle. In many cases, these fights have multiple distinct phases where the fight changes significantly, so everyone needs to learn how to deal with several problems in each phase. These fights tend to take at least several hours for a group to learn, and often, several nights of attempts. The progress is in knowledge gained of the fight's mechanics.
In a disorganized setting, the knowledge is lost. Many fights in the 4.1 raid content are simply inaccessible to pickup groups, not because they actually require anyone to do anything extremely challenging but because they require everyone in the raid to be aware of several mechanics in each of several phases, and to react to them (e.g. run away from the raid if you're the lightning rod, run to the fire if you have the ice debuff, don't stand in Corrupting Crash).
The primary barrier to entry for the raid game is the ability to show up at a regularly scheduled time, two to three nights per week. The recruiting pool on most servers isn't exactly deep right now, so if you can meet that qualification, a guild will probably teach you how to improve your throughput and help you to gear up.
Any encounter that requires substantial awareness or knowledge of fight specific mechanics is a progression wall for pickup raids, and any fight that does not involve multiple fight-specific mechanics is going to bore experienced raiders. Blizzard has several strata of organized guilds to serve with its raid content. The very top-tier guilds are comprised exclusively of the best players in the world, and they're willing to raid long hours to break a dungeon. The heroic-mode raids they see are incredibly difficult. There's usually a wave of hotfix-nerfs to heroic modes shortly after the "world first" competition ends, which makes that content accessible for hardcore progression guilds that aren't the best in the world. Below them are more casual guilds who tend to occupy themselves with normal mode raids, and comprise the bulk of the raid community. In Wrath, pickup groups were supported by having separate lockouts for 25-man raids and easier 10-man versions. Blizzard combined the lockouts and loot tables for 10s and 25s, because the raid week for casual-guilds was exhausting.
If you can meet the minimum criteria of being able to participate in scheduled guild raids, the raid game right now is very accessible. Widespread successful pickup raiding of current-tier content seems unlikely under the current design model. The 4.2 raid nerf seems to be designed to open up those dungeons to pickup raids. Blizzard has done similar things in the past, including the ICC stacking buff.