[guineapig]

I think the most difficult classes to master in no particular order are:

Enchanter
Bard
Warrior
Monk

I think when you are extremely good at those classes it stands out and gets noticed.

Other classes really have to do some pretty amazing out of the norm things to get noticed either because their utility in a group setting is limited or it's just a simple job function.

Reasoning:

Paladins and SKs have a very easy time managing agro when compared to warriors due to their spells.
Rogues basically just dish out damage and evade to avoid agro
Wizards... mcuh the same as rogues
Mages and necros: with a few exceptions their job in the group is limited to damage dealing. They can be called upon for crowd control occasionally which is when you start to see differences between the good and bad players.

(Stand out necros will also use mez, snare, root mana syphon, twitch and their limited heal spells. When you see necros doing all this in a group setting without being asked to, you should take notice.)

Rangers: Rangers do have some great utility but are often discriminated against for not being the best at anything. They have it rough simply based on common opinion.
Shaman: Tons of great things to add to groups and can even tank in a pinch. However they are often relegated to just buffing and slowing which doesn't often garner much praise.
Clerics: In a group setting their role is often extremely limited. You either keep people alive or you don't. Not to say that their aren't great clerics, but again they often don't get recognized as such.
Druids: Same as the above 2. In groups they simply don't get noticed for greatness.


These are just my opinions based on my years of playing in groups.

Enchanters, Bards, Warrior and lastly Monks tend to get the most praise for being awesome at their jobs.