- One or more humans gather within a physical, written, or auditory space.
- There is a story.
- There is at least one rule that dictates the behavior, actions, and/or thoughts of the individual(s) involved.
However, I'd argue that this type of definition for tabletop RPGs is only a more recent development. It would probably be the late 90's/early 00's when this paradigm shift really took hold and many, many indie games started popping up and change how we think about our organized make-believing.
So what's this all about? Well, nowadays we have a lot of ways to create characters for our games whether it's point-buy, life paths, or simply filling out a sheet with 15 questions. But back in the day classes ruled the world. I read RPG.net probably too frequently, and there are quite a few people there that just seem to rag on classes. Having not grown into the hobby playing 3.X edition D&D, I probably have a very different view on classes and levels than those who have been in the hobby for quite awhile. I respect the differing opinions, and I'm not trying to convince anyone to like classes (or really any other part of a game) but rather hope to express why I like classes.
I like classes because they're short hand for something that would otherwise take a long time to explain and build mechanically. When someone says their character is a ranger, my mind automatically goes to the image of a ranger in D&D. Either their character uses a range, non-firearm weapon or two one-handed melee weapons. He's wise in the ways of nature and hunting, and he may even have an animal companion. I know all these things share a common, mechanical foundation upon which their built, and so, much like many other disciplines and hobbies, we use such short hand because it makes things easier.
Speaking of easier, I like classes because they give me a structure within which I can make meaningful choices and know (hope) that the choices I make our balanced and have some heft to them. Don't get me wrong, this is by no means a rant against point-buy systems, but sometimes I just don't get a real strong feeling for what kind of skills and advantages my character will need if I just have a big list. It's a fine balance to strike between too much and too little freedom. To be fair, there is at least one great example of too many options in a class-based system as well - D&D 4e. There's just so many options there that character generation falls into an analysis paralysis of sorts (which is why I love Essentials so much).
I understand the limitations of a class-based system and why people prefer other character generation methods - mainly that class-bsed character generation feels too limiting. That's alright. I'm happy to agree to disagree on this topic. For my money, Warhammer Fantasy RPG nails classes with offering pretty much unlimited careers to choose from while giving you very few choices to have to make beyond your career. It's a system I strive toward emulating and matching in its balance between freedom and restriction for my own designs.
For what it's worth, keep it classy.