for me, an accurate start up procedure. for some crazy reason I love having to use a manual to start a bird up, until I memorize it. then I can brag to my friends that I know how to start up a F-111 8-)
As an option only please! (if you do it) That is one thing that would make me delete any FS aircraft - If I HAD to go through the procedure ... (each to his own)
For a middle ground, you could do what Iris does, gives you the option to either go through the full start-up or just flick a switch (usually a switch that has to purpose in the sim - such as the armament safety override switch).
