I have to jump in here and say that I wholeheartedly agree with SeaQueen's statements - graphics are nice to have, but it's all about the database and engine (AI, etc).
And of course I have to add my plug about having more driveables/flyables/operables in the sim. I can't play any game without wondering what it's like to operate the enemy sub/ship/airplane/tank/whatever. If anyone remembers Janes Fighter's Anthology, it was easily extensible in this manner. By certain manipulation of the save files and other methods I don't offhand remember, you could fly or drive practically anything in the sim - all the planes and most of the ground vehicles. Now it was lower fidelity than what we have here in DW but still kinda cool in a way
Playing a SAM (especially with a modified database) was the ultimate, given sufficiently crowded skies (easily done with the mission editor).