My collection of JG stuff is pretty small (hey, I am younger than most of these books), but here are my impressions:
The JG world, much like Greyhawk and Blackmoor, was influenced by different kinds of fantasy than most contemporary ones. In the 70s, there was no D&D canon, so all authors could base stuff on was Conan, Leiber's Ffahrd/Grey Mouser stories, Moorcock and Vance. As a result, the world feels different - not darker (it is high fantasy), but perhaps "harder" (as in "Hard fantasy" or "Black and white with lots of both"). It isn't PC, either: there is widespread slavery (you get rules for pricing), amazons in skimpy clothing and such...

Lots of weirdness, too - there are golems and harpies as wandering encounters in the City State, lots of ancient technology hidden here and there. There are umber hulk barmaids, a pub for trolls, lots of things which aren't present in common D&D anymore.
There are very few real states - most communities are stubbornly independent with little communication due to monsters. This also means most "commoners" have to be battle-ready, so no 0th level people there, or very few... In fact, the JG world is a bit like FR in power level, but it feels truly different and the overwhelming majority of high level types are either neutral or evil - hell, Lord Balarnega (the Overlord) himself is LE, and he is served by mind flayers and evil dragons!
The actual world description is fairly vague - you get maps (the greatest maps ever! - numbered hex grids, one for the players and one for the DM, or Judge as they were called!), and a booklet with locations of castles, their garrisons, local communities, monster lairs, weird sites and optional rules/random tables - for prospecting, taxation, generating lairs and caverns, etc. Lots of blanks you can fill in, lots of wilderness your players can explore (and map on their incomplete player maps!). For example, the City State included ten cool maps of dungeon levels beneath the city (or were some of these below Thunderhold), but you had to determine yourself who was down there. Or a bunch of caverns, with vague hints to inhabitants. Of course, the Necro treatment will be slightly more detailed... The JG staff was the ultimate master of cramming - with the City State, you also got a nice bonus in the form of a described little dwarven town and the nearby Sunstone Caverns. With Wilderlands of High Fantasy, you got Haghill, an even smaller town ruled by a guy called Huberic.
I love this world, and it might be the first non-homebrew I ever use.

If you like slightly crazy, slightly conanesque fantasy in a wild and expansive world, get the Necro version, or the originals, even.
