World of Warcraft is by far the most popular commercial MMO in existence. It uses a class and level system that is completely permanent. Unless the server gets rolled back, you will never regress in level, nor can you change your class without deleting your character or starting a new character.
The system used is pretty basic. There are a number of character classes (nine at the time of writing with a tenth to be released with the coming expansion, Wrath of the Lich King) that the character can choose from. Those classes are unique from one another with unique abilities and capable of using different combinations of equipment. Each class also has a slightly different mechanics for ability scores. A warrior, for instance, gets twice the attack power from strength that a mage or rogue would.
This system is good because each class is using different equipment, and each class is needed in it’s own way. It is balanced as each class has a role and even the hybrid classes cannot do everything. Everyone has equal opportunity, and no knowledge of the game is needed to play. The game is easy from the beginning, and gets harder as you play longer, as challenges become greater.
WOW does have a skill side, though. Weapon use and defense rating are based off of use. Every time you swing your axe, your axe skill can go up to the cap. Your weapon skill also determines how often and how hard you will hit. However, this is largely just an annoyance than an actual mechanice, as it takes about 15 minutes to level a skill from 1 to the max.
WOW also has an aspect called talent points. This allows a character to customize his or her abilities with small bonuses and extra abilities. For instance, warriors can increase their defense and attacking abilities, rogues can become sneakier or more combat oriented. The characters gain their own niche, and in theory every single character could have a unique talent combination. The problem with this is that there are still only a handful of talent combinations that are useful, leading to very similar characters.
I will be going more in depth with other aspects of WOW later. However, we see here that a class system is good for balance, and that it gives each character a niche. The game is easy to get into, and gets more challenging as you go. This is a game type that is good for beginning players and a more casual gaming experience.