Shouldn't this thread be about the merits of HarnMaster, or such? For some reason, whenever the name "Harn" pops up here, it always causes a controversy and ends up degenerating into a "my game can beat up your game!" argument. I'm not going to waste time going into that mess again.
I don't think this thread is (or should be) about which game is the "best", but about which game does what it sets out to do the best. These are two different things, IMO, and allows EVERYBODY to be happy. Finally getting the chance to read the "much-vaunted" HarnMaster rules for FREE was really an eye-opener for me as a steadfast d20 gamer (normally I don't like skill-based systems like GURPS or BRP), but HM, despite being a bit dry in spots, seemed to just "click" in the right places for my idea of a low fantasy game, so I gave it a more thorough read through than I thought I would.
Here are my observations, based on a comparison between d20 and HM (my thoughts on the rules neutral HarnWorld setting follow), and I hope I don't upset anybody, because a lot of posters seem so touchy and it's not my intent to contribute to any animosity!):
D&D is a great fit for moderate (GH, UA, Kalamar) to over the top (FR, Eberron, Oathbound, etc.) settings. It is fun, "everybody knows how to play it" and does what it sets out to do beautifully. I don't think I need to go into its virtues as this is ENWorld---we already know and love D&D/d20, warts and all! However, one thing I think we can all agree that it does not do well, is model low fantasy or historical settings, except with massive changes (Call of Cthulhu d20 being a sterling example), and I'm not sure it gets it right, even then.
HarnMaster, to me, clearly is a superior rules system for modeling what it sets out to do: an extremely realistic grim and gritty "low fantasy" game. It would work well with Thieves' World, Lankhmar, Middle Earth, or, I daresay, with the spirit (if not always the practice) of Conan. Combat is the scariest thing I have ever seen. Nobody in their right mind would choose to carelessly get into a fight using HarnMaster; the risks are just too great. Combat is descriptive, bloody and brutal. This engenders a more realistic, tactical and thoughtful style of play. When the players realize they must be creative and not resolve every obstacle with brute force or high magic, the game can get very interesting for everyone in ways that the "hack-n-slash/smash-n-grab" style promoted heavily (though not exclusively) by 3rd Edition D&D can rarely do. When every time you draw your sword or cast a spell may be your last, no matter how experienced your character may be (there are no levels or classes in HarnMaster, which is entirely skill based), it promotes a very different style of play. For those who want this sort of game, I recommend HarnMaster, regardless of whether they use the HarnWorld setting. HarnMaster does not appear to do high fantasy well. But if you want high fantasy, there are few better choices than D&D. We already know that, right?
After looking over psionics (I assume magic must work in a similar way, but it's covered in a different book, as is religion), the truly amazing part is how free-form it is. This is a rough example (Harniacs feel free to correct me if I'm wrong), but here goes: The player says, "I want to use this power of mine now" (rolls % and succeeds at a psionic power skill check). Rather than having a set limit to what said power can achieve defined by the rules, the DM would say, "What effect do you wish to achieve?" The player then states what he would LIKE to have happen, the DM then is free to assign whatever result, good or bad, he thinks should occur, within reason of the power and the success level of the character's power skill check. This seems daunting at first, like it would require a lot of extra thinking on the DM and player, but it seems to me that this keeps magic/psionics mysterious, and is far better at modeling fantasy literature and legend than d20's "cookbook" approach. Magic/psionics really can do ANYTHING (within reason, with the DM as final arbiter).
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HarnWorld, the campaign setting, is rules neutral and could be used with any system. It seems ideal for detail oriented DMs who can relax knowing that every last village and villager is detailed, as well as the world making sense, economically, ecologically, magically, culturally, militarily... monsters are very rare, and most are unique, and there is always a logical explanation for their existence. There are elves, dwarves and orcs, but they are kept mysterious and apart, nor are they balanced for PC races. It seems fairly Tolkienesque in this regard from what I can gather, but also heavily based on historical medieval England, right down to the feudal system.
It seems like one could easily get lost in all the detail, but I figure you only use as much as you want, and the rest is there if you need it. For a time-strapped DM like me, knowing that every little niggling detail is available at my fingertips seems like mana from heaven. "You want to go here? Ok, no problem--" ***flip***"You arrive at you destination, and here is what you see..." Nice.

I assume there's wiggle room to fudge things if need be and that the setting is not a strait-jacket, though changing one aspect too drastically might necessitate changing a bunch of other, related aspects??? I'm just going by the free stuff and previews online, so correct me if I'm wrong.
For me, buying a setting is to relieve me of the burden of coming up with my own. I want it super-detailed, so I can concentrate on adventures, not world-building. The more information available, the better I can make it come alive and make sense without breaking a sweat. Players constantly bug me for little details I have to scramble to make up. Let the eggheads sweat the details while I reap the profits, is my motto. Unfortunately, few settings have the kind of anal-retentive detail AND flavor I'm looking for. For example, I constantly have to remake FR fit my desires, stripping out tons of elements I despise, in order to concrentrate on the elements I do enjoy (which tends to make the extra work I endure worthwhile).
As an aside, I've been reading the Conan novels (Howard and others) lately, and really enjoy the flavor and real world parallels of the Hyborian Age. I'm a little peeved that the $50 Conan main book barely covers the setting and I must wait many months to get the Hyborian Gazetteer (now called ROAD OF KINGS and expanded to 200 pages, then wait more months for the Zamora kingdom sourcebook). Fortunately, the novels are very helpful in filling in details and flavor, but I shouldn't have to wait months AND read dozens of books just to properly get a feel for the various aspects and locations of Conan's setting. I am confident ROAD OF KINGS will deliver, as the d20 Conan guru Vincent Darlage is writing it, and Mongoose based the recently upped page count on how delighted they were with his work.
I am waiting with baited breath for the new Conan (d20) OGL game, which hopefully will go a long way toward remedying this shortcoming while still providing options for performing near-impossible heroic deeds IN MODERATION. If Conan fails to achieve this, I may very well try HarnMaster, as low fantasy is just what I'm in the mood for now. Doubtless, as all things are cyclical, I'll be wanting to hurl fireballs, fly through the air and fight legions of hill giants again sooner or later, and d20 will be my system of choice for these types of heroic mid to high fantasy adventures, because that's what it does so well and it's just so much damn fun.
