Runequest was a great system with some things that many sometimes felt were missing from D&D, like experience based skills (where if you spend time climbing, you get better at climbing, but not at swimming), a hit location system, a de-emphasis on magics dominant role, etc. We played the same game, same playstyle, in the same homebrew world I'd been running for years, and we had some good times playing Runequest for quite awhile.
When I first came across Runequest, I loved the fact that the rules made sense and seemed realistic, and I was young enough that mastering complex rules seemed fun in its own right.
But the implied Bronze Age setting did not resonate with me at all. I wanted a mix of The Hobbit, King Arthur, and Robin Hood, and it never occurred to me to simply
do that, but with Runequest's rules. (Again, I was pretty young.)
D&D just has this certain, je ne sais quoi about it.
I think it's worth exploring what this
je ne sais quoi might be. Monte Cook takes a stab at it in
D&Disms, a column he almost called "The Genius of D&D", and in the sidebar on page 106 of the 3E DMG:
D&Disms
I thought for a moment to call this column "The Genius of D&D" but I was afraid that people would think I was blowing my own horn. That's not what this is about. The genius (or luck, or both) that I am talking about has to do with the early days of the game's development, and nothing to do with 3rd Edition, except that we were smart enough not to change them.
In fact, without the concepts mentioned below, I'm fairly certain the game would not have survived long enough for there to be a 3rd Edition. The truth is, each and every one of them has contributed to the game's long life. I find it amusing that most of them are the aspects of D&D that get criticized the most, even by otherwise smart and well-informed gamers.
Levels
Often touted as unrealistic, the concept that characters go up in levels and gain power as they do is probably the best part of D&D. Put simply, the concept of level advancement keeps people playing. At any given time, when you or I are playing the game, we can look on the class charts and plan ahead -- dream, if you will -- regarding what new powers and abilities our character will get if we keep playing.
Without the "carrot on the stick" that's clearly displayed ahead of us (in the form of the level advancement system) we might not keep playing the game. If the characters never got better, or if "where we were going" wasn't clear, we'd get bored. And because level advancement comes as a "lump sum" instead of gradually, bit by bit, the benefits are almost always significant and impressive. Yet these benefits come often enough to be fitting rewards for consistent play.
Levels are also a useful measuring tool for the DM, when designing challenges for the players. They provide, quickly and easily, a shorthand means of gauging power. If the DM knows the players are 5th level, they'll probably lose if they have to fight a dragon turtle, but simply getting across a chasm provides practically no challenge at all.
Classes
Classes are brilliant because they provide focus and direction. While some people find them limiting, it is in fact those limitations, or the preconceived notions of those limitations, which make them so useful.
Classes facilitate the game as a group activity. If we all sit down at a table to play a game, and you ask me what my character is, I can go on for half an hour describing him -- and it might even be interesting -- but as we begin playing together, you need a shorthand idea of who I am and what I can do. As we go forward, and play the game over many sessions, you'll see that I am not "just my class" and you'll learn all the little details. But to begin with, you'll at least have something to grab hold of as a mnemonic, since you'll have to do the same with everyone else's character as well.
Classes channel character creation creativity. They provide a templated starting point for you when you make a character. They're not a straightjacket -- they are a median point from which a creative player can deviate. If you want a character who grew up on the streets but secretly wants to learn the arts of magic, you can create a rogue character and eventually multiclass into a wizard. With multiclassing, and skill and feat selection, you can create whatever character you want -- classes don't stop you at all.
Classes are simple. For people who aren't interested in in-depth character background, saying, "I'm a human fighter," or "I'm a half-orc cleric" is all the development they need. For many, development comes later, over the life of the character. Classes provide the luxury of not having to come up with every personality quirk, character backstory point, and area of interest right away.
Classes provide an interesting avenue for personal game design. Classes (and now, with 3rd Edition, prestige classes) are game design tools that DMs and players can easily use to tailor a character. While it's true that the skills and feats you choose can tailor a character, designing a whole new class isn't that hard, plus it's fun and can be very rewarding.
Classes also add to the longevity of the game in that they give players obvious goals for their next character. "Next time, I'm going to play a wizard." Or, "next time, I'm going to play a gnome wizard."
Hit Points
Sure, a system that describes being burned, falling from a high place, and being stabbed with a sword all using the exact same mechanic isn't very realistic. However, it's exactly that abstraction that makes the game playable and easy to learn.
Most importantly, however (and this is always overlooked), it works over the long term. You can play a campaign for years, and the system never breaks down. Threats always appear in their proper perspective. Whether you are 1st level or 15th, you know exactly how much damage "20 hp of damage" is, and what it will do to your character. Everything is clear and nicely proportional.
Virtually every other damage system is either too deadly over the long term, or not deadly enough. While that might be the desired result for the game in question (you don‚t want Champions to be very deadly, for example), it's not appropriate to D&D. Dungeons & Dragons is too broad in scope to use anything more complicated. The damage system has to cover all the aforementioned types of damage (fire, falling, etc.) as well as such varied situations as being attacked by a giant squid, being crushed in a trap, and even being shot with a gun (or a laser gun). Hit points, able to encompass anything new that a DM can dream up, suit D&D-style play perfectly.
Dungeons
I've already gone on at length on this topic in the sidebar on page 106 of the DMG. Let me just add to that by saying that not only are dungeons excellent learning tools for new DMs and players, but they're just lots of fun. Dungeon-style adventuring is escapist, simple and often lighthearted. It's not the be-all and end-all of D&D scenarios. The game can handle all sorts of situations, encounters, plotlines, or what have you. Witness such varied settings for the game as Spelljammer, Dark Sun, and Planescape. Nevertheless, the dungeon setting offers so many possibilities for fun gaming that to overlook it is shortsighted.
In Conclusion
Some people believe that D&D is the most successful roleplaying game* simply because it was first. I don't know if that comes from a lack of insight or sour grapes. Or both.
If you're reading this and thinking, "Well, I certainly don't need a carrot on a stick to keep playing, or a template to help me create a character for game X," or "I need hyper-realistic hit locations and different charts for different sorts of damage my character takes, as found in game Y," or whatever, all I can say is: Good for you. It's great that you've found a different game that suits you. But literally millions of D&D fans before you, after you, and playing the game right now have enjoyed the game for just these reasons (and plenty more).
Games need mechanics. They need tangible hand-holds for both players and DMs to use as tools to measure things that the real world has no means to measure -- personal power and skill, the severity of a wound, and so on. These facets of the game were all "sacred cows" when we worked on 3rd Edition. They are aspects of the game that we knew we shouldn't change. Hopefully, going forward, they never will change.
* It's the most successful RPG by far -- so much more so, in fact, that other roleplaying games don't really even register on the same scale.