This was the original argument in favor of attunements, which most people seem to agree with. The problem with the proposed solutions is that they don't address the issue of teaching people how to play in a raid environment. Getting an item or downing a boss only guarantees that a person went through a raid once and to be honest, the people you think are bad have likely been through raids more than once and are still bad. The only way to ensure that someone is at least adequate at raiding is to make them reach exalted rep with the current raid as well as downing all the bosses (probably multiple times) before progressing to the next level of raids.Raid groups are typically blighted by individuals whose knowledge of the game extends to blizzarding their way through a hundred heroic instances, and not much else. Players can turn up to the final raiding tier having completed nothing other than heroic five man instances (with no responsibility attached) and probably the worst raid instance Blizzard ever created in Trial of the Crusader. Many haven't completed any of the previous tiers, nor have come across the main mechanics of them or (worse) learned how to effectively play cooperatively with other people. In short, they are not prepared in either the gameplay or the social aspects of end-tier raiding and, essentially, make raiding a worse experience for everyone involved.
This is why I agree with Lulia that attunements are a bad idea - to accomplish the desired goal they need to be a long grind and if they are relatively easy then they are simply an annoying road block and don't actually address the stated issue. I also agree with the person that said the key is to keep the older content relevant (Felhoof?). Blizzard has already announced three changes that will go a long way to making older content worth doing: 10 & 25 man will drop the same loot, players will be limited in how many badges they can farm per week and shared 10 & 25 man raid lock outs (which pains me to say because I hate the idea of shared lock outs). Badge limit per week means that players can't quickly grind out a set of gear through heroics. Shared lock out means that if you're like most 25 man raiders and also doing a ten man of the same content, you'll now be doing your ten man in the previous content.
The only content that needs attunements are Heroic Raids and they already have it - you must beat the last boss of the normal content to even attempt it.