Anyone that understands MMOs and raiding can usually pick up on the basics of any class as they already know the fundamentals and utilize the class well.
Why it's always a joy to make alts with other ppl in your guild or other good raiding guilds on your server etc.
I've played SK, Cleric, Wizard in EQ1 raiding and Templar in EQ2 raiding among other various MMOs.
Any class can be played to a satisfactory level in a raid, that would be just doing your job.
Playing a class well is about understanding the game as a whole, once you understand the basics of Fantasy tank healer dps crowd control MMOs then each one falls into place. Someone that understands the game, the raids and the other classes around them will play better because they are attuned to more of whats going on around them, they are preparing to help with things they are predicting.
For example, someone said MT group healer was simple, target MT and press heal which was rightly shot down.
In a raid, MT group usually and when not I would be healing various people, not just otehr tanks but dps and utility too, watching the other healers, looking at who it is that needs help and then choosing which spell they most likely need just from their name in the raid window and their bars. Watching the other healers mana bars and looking for who may need assistance first.
One thing EQ1 was good for teaching good healers was prediction. Some nub dps may tell you healers are easy, but Im casting my heal for said nub dps before he realises he's about to take dmg.
Dps may seem easy and on the whole Id say dps is slightly easier than the rest in terms of fundamentals of the classes, they still have to react and be aware of things but not as many things and sometimes less important things yet sometimes just as important things, a nub dps can get a raid wiped for example.
Being good at playing a dps class isnt just about pumping out the most dps though, any good dps knows they should not be the masters of damage but the master's of aggro control. Doing as much as they can while staying in control at the peak possible safe aggro.
Tanks have to monitor the raid just like the rest should, can always tell a crap tank in a group because they simply focus on the mob and spam taunt while some add is on the dps or healer and the chat box is being spammed for help, its not till you run the mob infront of the fat barbarian fuck that he realises and promtly engages and aggro's it after the first mob died and your down to 20M ^_^
All in all, anyone thats good at raiding and good at MMOs is good at most classes and can play each or most classes to their max potential with their understanding of everything.
In EQ1 I thought Chanter was the hardest class to play, be a chanter and break into Fear when it was still normal content, or solo break into it with a small raid soon after it wasn't

Still funs!
In EQ2 I dunno really as many classes lost their potency from EQ1, many aspects were dumbed down or things like CC were spread around a lot in diff forms. Raid Healer, CC, Tank together with Raid DPS close behind prolly.
Still can't decide what to be in Vanguard tho =/