Graphically Aerofly FS2 is already very nice. Similar to DCS which brings me to the main difference between these 2.
There are presently no items at all which would increase immersion/realism a lot.e.g.
1. Sound cones. Engines sound the same regardless if you are e.g. in front or aft of the planes.
2. Audible buffeting for e.g. extended speedbrakes.
3. Windsound increase with extended landing gear / no sound during gear extension/retraction on the 737 and A320.
4. Touchdown smoke effect.
5. VC (and/or panel) shaking/buffeting at high AoA flight.
6. Vortices at high AoA flight.
7. A copilot in the A320, 737 and 747
DCS has the most realistic vortices I've seen in any sim and their use of HDR is brilliant.
Touchdown smoke is overdone but basically it's very nice. P3D has an even more realistic touchdown smoke effect AFAIR.
It would be great if IPACS would consider a few of these items for future updates.