| Adam Waggenspack |
There is wide variation of both Class and Skill based systems, and it is really more of a continuum than a dichotomy. Here are the definitions I generally see used for them:
Class-based means a system in which a character must select a class (or classes), and most (if not all) character abilities are based on the class(es) chosen. Many people use the term class-based to also mean level-based systems, although some distinguish the two, as there are class-based systems which do not have levels. The primary example is D&D, other examples include other D20 system games, Earthdawn, and RIFTS.
Skill-based systems are systems in which abilities and skills are used to differentiate characters. In general, any character can purchase any ability, subject to prerequisite abilities, ability costs, and the like. Skill-based is in some ways a poor name, as many class-based systems have some form of skills. Examples include Deadlands, World of Darkness, Runequest, FATE, HERO.
There are many games which do not easily fall within one of these categories. What kind of system a person enjoys most is a matter of personal preference, as both have their strengths and weaknesses. Ability to roleplay has nothing to do with the distinction between the systems, nor is there any inherent increase in realism based on the type of system.
I had a friend who converted D&D into an ability/skill-and-level based system. Stats were bought the same way, and he costed all class abilities, Base Attack Bonus, base saving throw bonuses, feats, skill points, and the like. Designing a spellcasting progression was a bit complex (as he had to deal with the issue of players designing characters that could cast a very small number of strong spells rather than the more typical range of spells), but a spreadsheet made it easier. It was interesting and a lot of fun to have that extra customization ability in what was basically a D&D game. Of course, he also changed many costs based on his views of abilities' value, so that you could no longer build certain base classes (cleric and druid were strictly weaker). He originally had a Base Defense Bonus that could be purchased, but from playtesting, he discovered that spellcasters would often ignore Base Attack, but EVERYONE maxed out Base Defense. After that, he had Based Defense be based on Base Attack.