Well, as far as I'm concerned, as a former FSX and X-Plane 10 user, all I can say is that FG3 amazes me each time I start it!
I admire the plausible autogen, IMO much better choice than that used in whatever algorithm X-plane 10 is using. Also the mesh, the textures, the highways and rivers... pretty much everything about it
Weather modeling is another aspect that trully makes FG3 unique! All users should explore it's possibilities, and as a glider pilot I welcome the ridge and thermal effects, way way better than what we can get with X-Plane 10, or even FSX + ASN. Just don't forget to enable the turbulence level in the "Advanced" launch menu. I believe this could go into the sim menu bar, somewhere under "Climate".
Then, the flight dynamics - my last acquisition for FSX was the Majestic Q-400, probably the most advanced turboprop ever built for MSFS. And guess what? The MSQ400 runs it's FDM externally using JSBSim! Does this sound common?