I've actually done this before. In fact I had a thread here before the crash about rating the classes. My idea was three different levels of difficulty: simple, experienced, and advanced. The classes break down as follows:
Simple
-Cleric
-Fighter
-Sorcerer
Experienced
-Barbarian
-Ranger
-Rogue
-Wizard
Advanced
-Bard
-Druid
-Monk
-Paladin
The reasoning is fairly simple. Clerics are effective even if they only heal stuff; nothing to do this round? Heal the party. Fighters can select passive feats like weapon focus, improved initiative, and toughness and just make attack rolls. Sorcerers are easy enough if they have passive spells like mage armor and simple spells like magic missile.
The barbarian requires a bit more finesse because he has more limited feats than a fighter and must be able to effectively choose when to rage. The ranger is more difficult for other reasons. They have more skill points than the fighter which can be a blessing and a curse, as well as spells (though they are few) but they have a clear fighting path to follow and it is relatively simple to focus on that. The rogue simply has so many skill points and special abilities that are less than passive that it takes a bit of know-how to play. The wizard is simply a more complicated sorcerer; fewer spells per day plus more spells known plus preparation means knowing enough about the game to make good decisions.
The advanced classes require an expertise in the skills of the game. The bard is essentially a multiclass rogue/wizard with specialized abilities which means many things to keep track of. Additionally, the bard is quite mortal with d6 HD and few abilities to protect it from damage unlike the rogue (evasion, trap sense, uncanny dodge, etc.). To add to that the bard's spells are quite limited but vital to a bard's effectiveness. Plus they require more adjudication that simpler spells like magic missile that deal damage.
The druid has a similar level of complexity. A limited list of armor and weapon types, plus an unusual spell list along with an animal companion and wild shape makes for a very complicated character. Mastering the rules and use of one of these abilities is not extremely difficult, but simply using one effectively does not make for an effective character. Being able to utilize all the abilities in tandem is paramount to taking this class on a successful adventuring career. You must always be aware of ways to use your natural surroundings with your spells, you must know when and when not to wild shape, and you must be cautious in your selection of magic items.
The monk is probably the most difficult class to play in the game because of its relative lack of options at low levels. A canny player must have a plan in mind when selecting bonus feats and must be adept at balancing his character's abilities since a monk requires decent scores in more abilities than almost any other class. Plus, many monk skills are situational and the player must be aware of his surroundings to know when they are useful. Things like Climb, Jump, and Tumble are great boons but serve no purpose when the character forgets about them. Additionally, the monk has more special abilities, both passive and active, than almost any other class in the game that must be remembered. Many a character has died because the player forgot about one of the character's abilities that would have saved him.
The paladin offers a significant roleplaying challenge to the player, who must balance a code of conduct that encourages clemency with a martially-focused class. Additionally, unlike the many feats of the fighter that are often passive, most of a paladin's abilities (smite evil, mount, lay on hands, spells) are very limited and situational resources and the player must know the best times to use them.
