I'll say it has to be the ubersniper ground AI/gunnery, as BC said, the sim is head and shoulders above the rest, and its almost a shame to have a gripe about it. But then again, if it was perfect, well....I guess there'd be no need for SOW: BoB then!
The AI has to be the one thing that stands out for me as the biggest 'spoil factor': The prime example which will forever stand out for me is the time I was testing the new Far East map with a flight of P40's - I had been airborne for around 45 minutes, enroute to an airfield, altitude of 4000m.
There I am, admiring the scenery, its a beaut day, when CLICK! The lights go out - !
Upon replaying the track - There was a solitary AAA in the city below me - his very first round exploded literally in my face, killing me instantly! My aircraft had no visible damage, and carried on flying for several minutes before gently crashing into the countryside.
Many more examples of this kind of thing, but that one stands out the most!
Otherwise, iL2 rules the roost!!