As people have mentioned, the 109's just sound spot on. I've even made myself a mission in the FMB where there's a static camera on a runway and I go up in a 109 to buzz it then switch to the camera just to enjoy the sounds.
Other than that, the satisfaction of keeping an aircraft in the air without the engines exploding (nothing beats the Blenheim for this. It's my favourite aircraft in CloD just for how rewarding it is when it all goes right) and most of all, properly navigating. I never really tried it on Il2, but the satisfaction of working out a heading, flying to the target in my Blenheim and then seeing the target appear, knowing I got the heading right, is extremely rewarding. Especially if I can find my way home afterwards. Also when I get lost in a dogfight, trying to find my location and barings based on what I can see is great fun for me, especially if fuel is getting low.
The best thing though, as people said here, is getting shot at. When things start to go wrong and you turn for home, those times when the white cliffs surely couldn't be approaching any slower, you're losing altitude and your engine is screaming at you, those are the best parts of CloD I think. All the better if you make it back, but still amazing even if you don't.
EDIT: Actually, as Carguy said, the gauges are also a brilliant part. I'm actually learning to use them properly. Since, as I said, I spend most of my time in the Blenheim I've got used to what readings should be considered 'normal' and can now fly the aircraft with only glancing at the instruments every now and then. It's also the first place I turn when trying to figure out what went wrong when I get shot at, and if I'm about to plummit earthwards or not
