One of the oft repeated rubrics by which the quality of super hero games is measured is what I'll call The Green Lantern Test. Many consider the ability to adequately model the Green Lantern a key indicator of a SHRPG's flexibility. The thinking is, of course, if the game can allow you to create a playable GL, it can likely do just about anything else you need it to in terms of character generation. In the universal sense, it's not a perfect test. For example I'm fairly certain that one could adequately model GL in Mutants or Masterminds, but, given my limited intellect, I would still find it unplayable. However, if one likes the game play of a certain system and it can model GL, chances are (keeping in mind no game is perfect all the time) it will facilitate a benign SHRPG experience.
When it comes to Sword and Sorcery or Science Fantasy RPGs, GL's role is perhaps most properly assigned to Conan. Conan is at various points in his a career, a warrior, a thief, a sailor, a bandit a forager in the wilderness and many other things. Class and level based games have, by and large, two distinct ways of dealing with Conan.
The easiest method, at least in terms of book keeping, is best exhibited by OD&D 1974. Essentially everyone is either a Fighting Man or a Magic User, and the actions of players outside of their two main oeuvres are constrained only by the imagination of the player, the ruling of the referee, and the parameters within which the group can collectively suspend disbelief. As a referee I am perfectly content with this system. In my experience, players, however, like to have options. They want their adventurers to differ from one another. Occasionally, despite, the repugnance of it all, a conscientious referee must take the wishes and desires of players in to account.
This leads us, as it did the earliest formulators of the game and their successors, to the second method, found in the supplements of OD&D and every iteration of the game released henceforth: a proliferation of character classes, e.g., Thief, Fighter, Ranger, the noisome Cleric, and so on. Many of these classes model one or another part of Conan's career, and, quite obviously, go well beyond its boundaries. None of theses classes, however, adequately model Conan's entire career.
I would find this frustrating enough if the problem stopped with Conan. It does not. What if someone wishes to play a character very much like the Grey Mouser? What class should the select? The facile answer is, of course, under 1e rules, a simple Thief. I would have to disagree, though; Thieves do not fight so well as Fighters, and I never felt that the Mouser was any less of a swordsman than Fafherd or, Conan, for that matter. What if a player wants to emulate Severian from the Book of the New Sun or that Beast Master guy from that 80's movie of the same name?
Setting aside the straight OD&D option for the reasons mentioned above, how can this problem be solved, without resorting to the semi-solution of cumbersome and fiddly multi-classing? Furthermore, as has been pointed out by others, as soon as an ability (e.g. pick pockets) is given to a specific class, it is more or less taken away from the other classes.
I submit, therefore that the proliferation of character classes is an inadequate solution to the Conan/Grey Mouser/Severian problem, and, furthermore, as a phenomenon, introduces more problems to the game than it solves.
These problems have stalled my own design efforts, bringing me, in reality to a dead stop for the last couple of months.
Yesterday, I cracked the code (to my own satisfaction, anyway).
Before you continue, keep in mind that I still view OD&D 1974 (or S&W Whitebox) as nearly perfect just the way they are (the way they are practically necessitates house rules, but that's another conversation, altogether) and all my Metal Earth rule modification are modular, and any adventure material I create, or have created, can be used with the additions or without them.
The admittedly nascent system proposed below is linked to some of my earlier ideas, namely saving throw based task resolution. If you don't want to read about the stuff, just keep in mind that I use a simple system of saving throws and modifiers for non-combat task resolution. Just by reading that sentence you have been introduced to about 90% of that system.
The Fighting-man class has been, in my Metal Earth game, replaced by the Adventurer class. Other classes will be the Sorcerer, and, possibly the Mastermind.
I know it's unseemly to beg, but some comments on anything you've read here today, would be smashing .
Anyway, here it is:
A member of the Adventurer class can be a Wanderer, warrior, thief, sailor, soldier or, at one time or another, all of the above and more. The ability to survive lies at the core of this class.
At first level and every 2 (3rd, 5th, 7th) levels thereafter the members of the adventurer class may select one competency from the list below. For the purpose of skill advancement, the level at which the character picks a competency is treated as first level when calculating modifiers. For example if Kronark the Barbarian selects the thief competency at 5th level, this level is considered first level when calculating all die rolls).
Offensive fighter: Offensive fighters gain +1 to attack rolls and a +1 to all damage rolls- including results on critical hits tables.
Defensive fighters: Defensive Fighters gain a +1 to their AC (unless taken by surprise) and a +2 to all saving throws vs. wounds/critical hits.
Badass: A Badass character can deal lethal damage (including critical hits) and receives a +1 to hit with unarmed attacks.
Advanced Badass: (Prerequisite, Badass or Defensive Fighter): An advanced Badass has an automatic 4  AC (against melee based attacks) without armor.
Survival: Characters with the survival competency gain a +2/level to all survival related task resolution saving throws, including foraging and overland navigation.
Sailing: A sailor knows how to handle themselves aboard a ship. This includes competency at basic ship board tasks, such as getting under way and stowing cargo, and setting sail as well as more advanced tasks such as navigation.
Thief Skills: A character with the thief skills is adept at such activities as picking locks, climbing walls, and picking pockets. The character receives an initial +2 and +1/lvel to all task related saving throws whilst using these skills.
Marksman: A marksmen receives a +1 bonus to hit and to damage with firearms (including critical hits at close range).
Torture: Character that have been schooled in the art and science of torture receive a +2 per level on all interrogation based saving throws.
Art: An artist is especially skilled at some sort of creative expression. While in a city performers musicians, dancers, jugglers can earn 1d20 Zorms a week. If they are left to depend upon their creativity for sustenance, other artists will almost certainly go hungry, unless they can somehow arrange for a wealthy patron to subsidize their lifestyle.
Beast Mastery: A beast master has way with animals and can effectively charm Lvl +2 hit dice worth of creatures.
Keep in mind these are just samples, and the players and referee can easily create their own sets of abilities.