Immersion would be the my highest priority. Sure graphics and flight models and historical realism, AI and all the rest need to be 'good enough' but I'ld rather lower the visual settings to allow a more diverse environment.
A lot of the immersion comes from mission design, and I have a great deal of respect for the members of the community that are making missions and running the multiplayer servers. One of the biggest priorities must a set of user documents to help these guys out with their scripting and mission creation. When you think what these guys have done without the detailed documentation, imagine what they will be able to do when they have access to all the relevant information!
