Well, to be fair, Greyhawk was the premiere setting. Forgotten Realms just took off because it catered to massive amounts of power. Krynn is the world of Dragonlance, which is one where it's much harder to progress (you don't find a +5 vorpal sword lying under every rock like you do in FR). Planescape is the wild and crazy one that requires a very creative DM. There's more, of course...
(I was a massive DnD nut back in the day, so lease forgive me for running with this!)
Greyhawk, was just, Grey. ;) Setting was bland, and not as well supported as the others. But it was the original. Just not the best! :P
Krynn really only had support due to the (awesome) novels. Although the later Dragons of Twilight sucked. ;) You couldn't really play in Krynn, and if you got to 20th level, you were killed or became a god.
Planescape was a nice idea, but as you said quite difficult to run.
Can't remember the name of the new setting (that DDO uses), with Waforged and all that.
Ah! Dark Sun was another. A sort of post apocalyptic setting with Half Dwarf Gladiators that could crush anything. Problem was you spent most of your game trying to find water in the desert. Not really fun.
Of them all, FR had by far the largest suppliment support, and I'll wager novel backing as well.
While the other setting were mostly niches FR was the most traditional high fantasy setting of them all, and was unbeleivably rich in setting. The sheer amount of Gods they had (that you could worship as a PC) was staggering. On top of them they had just as many 'named' characters as the other Worlds (I actually forget who wa sin Greyhawk...). With Elminster to go toe to toe with Raistlin (Who was the third mage Dragon Magazine roped in?), and some memorable series to temp players away from the Dragonlance. The Godswar, Pools and Spellfire series for example. And that's not mentioning the lands that mimicked other lands here. Like the Celtic Moonshae islands, or the more 'oriental' lands.
The power level in FR wasn't as bad as you implied, but it was high fantasy. Especially when you had people like Manshoon and his 9th level spell Stasis Clone to contend with (every cast created an exact duplicate of himself with memories up to the point it was created. Held lifeless in never aging stasis. The clones memories could be updated with a touch. When Manshoon was killed, the next Clone in line woke up.), but then of course you had the Paladine like Elminster mooching around.
And Spellfire. ;)
I'll leave you with one of my favorite introductions;
"I'm Torm, and these are my feet!"