Hiya!
Hmmm.... for me, the best 3 things about Greyhawk would be:
(1) It's
GREYhawk. The three morals of alignment, Good, Neutral and Evil are all there. However, Neutrality seems to have the 'most widespread' influence. Most folk in GH just want to live their life in peace. Alas, Good wants to "make things better for them" and Evil wants to "make things better for themselves"...so there are a lot of skirmishes, border wars, etc going on somewhere in the Flanaess (the name of the section of Oerth [the planet] that is "Greyhawk") all the time. Because of the pervasiveness of Neutrality, guys who are really good or really evil tend to stand out (re: paladins, anti-paladins, devout clerics, dragons, demons, angels, flumph, etc). If a commoner saw a tiefling walking down the street, you can bet your bottom copper that tiefling is going to get noticed...NOW! In keeping with the "greyness" of GH, the druids actually play (or can, if you so choose) an equal, maybe even greater, role in the goings on of the world. You may have the generally Good forces of Furyondy battling the generally Evil forces of Iuz...but you *know* that the forces of Neutrality are in the middle, playing both sides. This makes for, IMO, a very "Game of Thrones" kind of feel, with armies of human militia making up 95% of any force (generally speaking). It brings it all down to earth, making the fantastical side of D&D, well, fantastical and not just "Oh, look, a contingent of Dragonborn Sorcerers riding wyverns."
(2) It's a buffet. Take what you want, leave what you don't. I've run multiple campaigns of multi-year length in GH since my first exposure to it with the '81 "folio". Each and every one has been different...sometimes vastly so. I've used 1e, 2e, Hackmaster 4e, and even some non-D&D-style systems (Powers & Perils, and HARP). I've had campaigns set in/around the same area as a previous one...with totally different feels to 'em. As a DM I don't have to "change" much of anything...mostly because what is there is loose enough for interpretation or usable from different angles. For example, in the Yeomanry; one campaign was focused on raising and training militia for a noted up-tick in Giant activity (sort of a military-feeling to it all)...and another campaign was focused on discovering the epicenter of a huge, mysterious pinkish-red 'explosion' that covered half the country one night (a very 'magical weirdness mystery' style feeling). Both took place (game year wise) at the same time. I could use the info given on the Yeomanry to run both. I didn't have to "ignore" anything, nor did I have to remove NPC's, or make assumptions about anything...I was free to write what I want "around" the skeleton of it all.
(3) Community. I am bias, but I find the old grognards who stick to Grehyawk easy to talk with (for the most part). This may be because I'm in the same rough decade as them (I'm 45). Ask a question about GH and you'll get 8 answers of "well, this is how it is in MY campaign", 2 answers of "well, in pre-wars...", and 1 answer of "well, post-wars...". But you pretty much never get "LOL! Newb! Go read the books, it's all right there...duh!". Also, if you ask for ideas for your campaign where the Circle of Eight were killed off or otherwise disappeared decades past, you'll get a lot of what you asked for: ideas. You won't get people telling you that you "aren't playing
real Greyhawk", or that "you'll have to rework so much lore it won't be worth it". I guess the closest I can think of is...the GH community seems overall very supportive of other DM's campaigns (exceptions there are...after all, lots of old dogs in the kennel and all that...).
Bonus Answer! (4) Anna's maps. Just looking at these things....*drool*.... If looking at her maps doesn't get your creative juices flowing, well, there's no hope for you, sorry. Oh, here...:
http://ghmaps.net/
Bottom Line: If you want a game world with AMAZING maps and descriptions that spark your imagination, Greyhwak is it. If you want to be able to run campaigns from gritty war-stories, to high-magic save-the-world campaigns, Greyhawk can accomodate. If you want a world where the players feel like their PC's actually
are the "only ones who an rid the town of the ogre menace", the Greyhawk will do perfectly. If you want a campaign where the players feel like their PC's are just entertainment for the cabal of high-level NPC's that sent them to "rid the town of the ogre menace" (knowing full well that even just two of those NPC's could deal with those ogres in the space of about 20 minutes)...well, Greyhawk probably isn't for you.
^_^
Paul L. Ming