Tobold recently posted an interesting response to a question posed to him regarding how to begin raiding. While I agree with him in that the current implementation of raiding seems somewhat wasteful toward resources as raid instances only receive heavy use while they are the current tier of content in WotLK and then virtually no use after that point, I disagree with his assessment this is a failure on Blizzards part. Today there are actually many people playing that never raided Naaxramus when it was the current tier of content released in its current form less than two years ago at the launch of WotLK in November 2008. But they don't need to now and that is testament to the fact that Blizzard acted on the criticisms it received by many regarding the high cost in terms of time and effort to break into raiding, as well as progression and accessibility issues through the end of BC.
Each new expansion of WoW has brought further openness and greater access to game content for players. Today we have a game that makes it vastly easier to begin raiding and access end-game content than at any other point in the games history and in fact I think we're at a point where we've drifted too far in that regard. It takes virtually no effort on a players part to begin raiding which only serves to engender a mentality of instant gratification. What we see in WotLK today was a result of criticism Blizzard received from players in Vanilla and BC where raiding end-game content was by no means easy to break into and did require considerable time and effort to advance. In BC you absolutely had to raid Karazhan to obtain gear before you could go onto raiding Gruul's and be viable. But you could only reliably raid Karazhan after weeks or even months of funning heroic dungeons trying to assemble your "dungeon set". You then had to continue raiding Karazhan and obtain gear off the final bosses as well as gear from Gruul's Lair before you could reliably raid Magtheridon's Lair and you needed gear from Gruul's Lair and Magtheridon's Lair before you could reliably raid in Serpent Shrine and Tempest Keep. And you had to obtain gear from Serpent Shrine and Tempest Keep before you could reliably raid Black Temple. And finally you needed Black Temple gear before you could reliably raid in Sunwell. Ignoring the key and attunement requirements that were also involved in all this, and that were removed from raiding mid-way through BC, the requirement to raid previous tiers of instances before moving on to the next tier had been a hallmark of end-game since the inception of WoW.
The problem with that then, and remains a problem today is that people who raided the content when it was the current tier of content have little, if any, incentive for going back and raiding previous tiers again after they've moved on. New players found it just as difficult in BC to break into raiding after a certain point as they did in Vanilla. That isn't nearly as much of a problem now as it was in BC as there is an alternate means of gearing up to the current tier of content that largely solves this problem. If this alternate system wasn't in place a players only means of obtaining gear would be through the tiers of raiding content and would require finding groups to raid with who were at the same point in progression. Obviously that was the problem in Vanilla as it was in BC and would continue to be a problem in WotLK as well were things not how they are.
If you were to hit level 80 today and wanted to begin raiding there are a number of ways in which you can quickly get geared to the point where you'd be a viable raid member. But before beginning to raid you have to understand that WoW is a social game and raiding is a team activity. like any group activity such as this you're going to have to accept that your other team mates have expectations of you and of everyone else in the group. Even if you think you are ready to start raiding you might not be accepted into every group based on entirely subjective judgments about your class, spec, and gear. However that being said, getting geared up to the current level of content doesn't take very long at all. Your first option is to obtain craftable pieces (bracers and boots) and purchase BOE pieces like belts and chest pieces off the auction house. Because of the expense this may or may not be a viable option for many. Luckily there are other options for obtaining gear. Any new character should run VOA religiously every week as there is an opportunity to get T10 set pieces like gloves, pants and helm. That alone is worth it's weight in gold as any piece you get there can potentially save you three weeks of running the daily heroic, weekly, and VOA to get your first piece of T10.
Beside earning two Frost Badges from the daily heroic run, and the five you can earn from the weekly there are four heroic dungeons (Trial of the Champion, Forge of Souls, Pit of Saron, and Halls of Reflection) that drop ilevel 232 gear which is plenty good enough to get you into some TOCs if not an ICC-10 where you might be lucky enough to pick up some much needed loot. Even if you didn't earn any loot from a random run the badges would be reward enough because with those come your other T10 tier or off-set pieces. In short raiding progression is currently designed the way it is in WotLK for specific reasons and it will take some level of effort on your part to get ready for raiding. But adequate means are there for you do that if you apply yourself.