If you don't mind me asking, why are you sticking with fs9 so long? CPUs are fast enough to run FSX (today) better than they could FS9 (pre-FSX). I'm not knocking FS9, just curious.
AI. I cannot imagine flying in FS without AI. No matter what I try in FSX, I cannot get a fluid flying experience with 100% AI and most of my aiports are filled with close to 100% real world traffic that serves a given airport.
I have a fairly decent machine (i7 950 @ 4Ghz, 2 GTX 285s in SLI) and yet whenever I approach a major airport in FSX, it ineviteably stutters and convulses making any sort of landing nothing short of impossible.
The difference in FPS between, say, FSDT's JFK in FS9 and FSX with the exact same AI present is nothing short of staggering, I get between 30-35fps on approach in FS9 vs 9-12fps in FSX. Unfortunately for AI nuts, there still are no machines fast enough.