I think the lists people gave for civil flight simulations apply very good for what modern flight simulations should include. In fact, these kind of details will attract a lot of people to flight simulation again.
What a lot of people don't understand is the fact, that the flight simmers are a huge bunch of people who are desperately seeking for the ultimate simulation. And this is the state they/ we are all stuck in since years!
Take the civil flight simulation market for example. What people have is X-Plane and MS Flight Simulator. Now, Im not an expert in these simulations but I can tell you that for everyone who is interested in flying has to chose between these two options. MS Flight is a big overloaded donkey, where people have put so much stuff and addons to, that the donkey finally gave up walking. It's overloaded and doesn't offer any of the details I or any of my flight sim fans look for.
Then there is X-Plane. Well, it's X-Plane and as far as I can remember it has the most ridiculous physics I have ever come across in a flight simulation.
To understand what flight simmers really need, it comes down to the things Kelvinr and others have posted already. It's about looks, physics and expendability. If an engine is able to provide these basic modules, it's an instant win.
But I would like to speak for another group within the flight simmers, that I think is a little underrepresented here. Because I'm actually not interested in flying a Cessna from A to B, looking to the left and right. It's fun, yes and sometimes it can be very educational.
But I am into the military flight simulation all the way. And let me tell you, there are
so many military flight enthusiasts out there, it's incredible. Military flight sims were the first games so many people were confronted with in the old days. And they have grown up. So did technology. But the simulation genre didn't for some reasons. Now what people still stick to, are games that are totally outdated. Like Falcon - which is an incredible simulation but just grown out. There are addons like BMS that still try to keep only the Falcon community alive and attracted. I've been playing this simulation for years and I love it. But it's time for a real game changer. People thought, that DCS might be able to do it - but they didn't.
So I think, that the military flight simulation could boost Outerras development to a whole different level and the fact, that it is able to process data so detailed will let every mil-flight-sim fan fall in love immediately.
In fact, I have thought about combining the civil and military flight sims, +war simulation and strategy/ economy into one big sim and let it run online in a "persistent world". ATC's could be implemented, diplomacy and strategy ... the perfect next-gen MMO
I can go on with my idea - whoever might be interested, I can offer a book of ideas!
So - flight simmers need lots of things. But it becomes truely amazing with military simulation. It's like a beer. It's great, but ultimately amazing when it's cold.