Hmmm, I may be in a minority here (and, yes, I realize that I usually am), but I think much of the debate over D20 vs. D&D is one of semantics.
D20 is, in its broadest context, "the current game"; D&D, however, is a subset of D20. D20, for example, also includes D20 Modern, Arcana Unearthed/Evolved, Sidewinder, Conan, Star Wars and a host of other products all using
similar, but not
the same, rules.
I currently use a set of D20 rules; I have not played D&D in a couple years now. I have never been overly fond of several of the D&D core concepts -- I am speaking here in terms of setting, rather than rules. Greyhawk and Forgotten Realms (both D&D) never really sparked my interest, I dislike the framework of dungeons (mainly because I cannot fathom why they were constructed in the number and size they appear to have been in many D&D-based modules), and I am not overly fond of many D&D-specific monsters. Conversely, I find the D20 rules, as a set of base mechanics, easy enough to explain and generally popular.
It is also very easy to start a D20 or D&D game simply because it is ubiquitous. They show up regularly in ordinary bookstores (many, but not all, of which also carry GURPS and World of Darkness).
Let me compare rpgs in general to card games for a moment, simply to make a point. D20 (or possibly D&D) may be likened to poker. Everyone who knows much of anything about cards knows at least the basics of poker, though there are so many variants it can sometimes make your head spin. Some people will only play Texas Hold 'Em, while others will play anything that is called poker. This is a matter of personal tastes. Then there are games like bridge, cribbage and rummy; these games might be likened to GURPS and other popular non-D20 systems. These games are not as widely known, though many similar elements are there (the cards); still, enough people know of these games that they are playable in the wider community and they can be taught with a minimum of fuss. Now a gin rummy game would never be confused with a seven card stud poker game, but at least we would all recognize them as being card games. Then there is tarroc. This is also a card game, but it is played with a tarot deck and has not been widely popular since the 16th century. Totally different set of cards, very different set of rules, there are many people who would wonder what the heck you are playing and would never even bother to try and learn the rules since it is so differnent. This is the place of All Flesh Must Be Eaten, Ars Magica, Paranoia, and many other games out there -- so odd that many people never learn what they are all about.
The point is that despite the surface differences, poker, bridge, and tarroc are all card games. Some people have a strong desire to play one and refuse to play another. Fine. some people will play anything as long as it is a card game. Fine. Some people will even want to learn about a game that they do not play, even though they have no intention to ever actually play it. Still fine. The problem is that many people will go out of their way to decry the card games that they do not like while crowing about the game(s) that they do. This is, obviously,
not fine.
Okay, D&D is not my Game-of-Games. On the other hand, I would gladly play it if a buddy of mine was running it. Heck, for that matter I would probably play any rpg that a buddy of mine was running, simply out of sake of friendship if nothing else. I run a D20 game that I have modified to suit the tastes of both me and the gamers who sit down at my tables. That's good. We are happy with it and we don't need to justify that happiness to anyone else. And when we switch in the upcoming months to a game that is not D20 we shall not look back on our D20 times with scorn or derision, but rather remember the great times we have had with it. And if we return to it again someday, we shall not talk about the times we wasted playing Ars Magica, but again remember the good times with that game as well.
RPGs are games. They are a hobby. They are not something to start a fight over or to get bent out of shape when one person likes one game and somebody else likes another. There is no need to justify playing the games in general or to justify the choice of one game over another. They are fun. Some of us take are games very seriously, some strictly as a lark. But in the end, everyone has their game and that is what it is all about.
Do I play D20? Yep. Do I play D&D? Not currently.
Do I play roleplaying games? You better believe it!
