I've seen countless arguments as to why vanilla raiders are supposed to be better than those who started late in the game. Being one of those players, I have to agree to an extent. Here's why: haste.
Haste was abruptly introduced sometime in TBC (it was either during BT or Sunwell I believe). It became the reason why Sunwell locks did literally nothing but press Shadow Bolt. It became the force behind Chain Heal not being useless. Fast forward to late-WotLK, and haste is the godmode stat. Every single caster loved it like crack. Especially shaman healers. High crit ratings on gear meant absurd returns on IWS, so of course haste was the best stat choice for output since mana was a nonissue. Fast forward finally to Cataclysm, where mana IS an issue. Haste has all but bottomed out for healers as a stat choice. Second only to mastery for resto shaman as the most useless stat.
Getting to the point of all this: those of us healers that lived the "glory days" should already know how to heal effectively without haste. Those who joined the game later, haste has always been there. Now some of those healers are still using it and wondering why they're OOMing seemingly too soon. With Cataclysm's healing model, proactive healing plays a much greater role. Starting casts in anticipation of damage, as well as paying attention to overheal, is a vastly different playstyle than haste-kiddies are used to.
So, in effect, us veterans have a head start on the curve. Sure, there are probably many players out there who started late but made the adjustment fine. However, I'd wager that's the exception and not the rule.
Thoughts?