I'd say alomost any setting is a good setting, but it's only good for you based on what you & your players demand from it.
Eberron is cool, & has the higher "tech" level in comparison to other D&D settings. Bit of a FF feel to it, IMO, mixed in with a bit of Indiana Jones-style.
Forgotten Realms is nice, but I'd say you'd have to tailor it to what you want, since there is SO MUCH MATERIAL on the setting.
Greyhawk, IMHO, is always a good option. Plenty of current adventures to use for the game (like the Adventure Paths).
Nehwon (the setting for Fafhrd & the Grey Mouser) is a great setting, though it's OOP and would take some eBay hunting & some small degree of conversion work to bring up to the current edition of D&D. Here's a quick, down-&-dirty rundown on the setting:
Low magic: magic items are few & far between (and so rare that they'd bring undue attention to the owner). Items as a reward probably should be masterwork or unique materials rather than magical.
Earth with a touch of Weirdness: There's a slight hint of Cthulu in Nehwon (but not to the extent of driving the heroes to madness, only the unlucky NPC or villain of the tale). Most foes should be NPCs, animals (and dire animals), and maybe the occasional aberration, ooze, or such (as a very rare occurrence). Dragons as in D&D are unheard of, as are many classic D&D beasties (there are dragons, IIRC, but they're more akin to a magical beast at best, or a slightly-smarter dinosaur variant). Undead are to be feared (esp. since none of the classes recommended for Nehwon can turn undead).
Heroic Luck: Eberron's action point mechanic (also in
Unearthed Arcana, and thus mentioned on
d20srd.org), would be great for the setting.
Alignment: At the least, you should go with the take on Alignment used in Eberron--Nehwon is a morally-grey realm at best.
Humans only. There are some unqiue races (human offshoots) in Nehwon, like the Nehwon Ghouls & Invisibles of Stardock, but they'd be best suited for NPCs rather than regular PCs.
Classes in Nehwon: The only magic users are wizards. Non-spellcasting variants of classes & prestige classes such as rangers, bards, assassins, & the like should be used. Here's a list of viable classes for use in Nehwon:
- Barbarian: as is. Most likely encountered in the North (Northerners), East (Mingols), or South (Natives of the Jungles of Klesh).
- Bard: Non-spellcasting variant only (if you can find one). Or, just have a bard-themed Variant Rogue (possibly getting Bardic Knowledge in exchange for Sneak Attack or the like).
- Fighter: as is. Common everywhere.
- Monk: Can work, but should be few & far between.
- Ranger: Non-spellcasting variant only.
- Rogue: as is. Common everywhere (esp. in cities).
- Wizard: Works for the "black wizard" of Leiber's tales. Always corrupted by their magics: the taint rules from UA (and at d20srd.org) should work in conjunction with the normal class. Slap on a nonGood-AL only restriction on the class & it's good to go as well. However, the old versions of the D&D treatment had adopting an Evil AL by 5th level mandatory for continued progression as a "black wizard". You could include this, if you want.
- Wizard (Elemental Specialist): Basically a wizard who's bound for the Elemental Savant PrC. Can be of any AL (IIRC).
- Swashbuckler (Complete Warrior): Good option. Common in cities.
- Scout (Complete Adventurer): Another good option (maybe to replace the ranger, even).
- Erudite (Expanded Psionics Handbook): Few & far between: mainly from the underground realm of Quarmall, and a trait of the ruthless ruling class. I think Erudite, rather than psion or psychic warrior, would work best for psionics in Nehwon.
- Knight (Player's Handbook II): Can fit in well (use as a replacement for paladins).
- Archivist: From the Heroes of Horror supplement (and available online at the WotC site). Best represents the "white wizard" of Leiber's tales. Slap on a Good-AL only restriction & it's good to go.
- Human Paragon (Unearthed Arcana/d20srd.org: Seems like a great boost for any heroes running around Nehwon, and can work for the blacksmith or young noble who decides to take up a life of adventure (though they'd have to shift into another class later).
PrCs should likewise be limited: variant assassins (sans spellcasting) should be used, & PrCs should not grant spellcasting on their own (only add to an existing core class's spellcasting power). Black wizards should go for more sinister & weird-themed PrCs, like Acolyte of the Skin or Alienist, while white wizards would best be suited for non-combat oriented PrCs like Loremaster. Other PrCs good for the setting (off the top of my head) would include Duelist, Invisible Blade, Master Thrower, Dread Pirate, Extreme Explorer (from Eberron), Exotic Weapon Master, Street Brawler, Raging Berserker, Evangelist, & many more.
OA-themed classes like the samurai, shugenja, ninja, & wu jen really wouldn't fit in to a Nehwon campaign (no real Japan/China analog--the primary Asian-based culture in Nehwon is the Mongol-like Mingols).
Of course, you could modify the setting to your own specs & prefs, but this is a general approximation of what the setting's like.
