Just to get this started:
- FlightGear doesn't cost any money, it can be freely downloaded
- FlightGear is being developed by volunteers all around the world
- FlightGear uses no proprietary binary file formats, it uses plain text (XML) for most of its own configuration files
- FlightGear doesn't need FSUPIC, all its externals are directly available via a bunch of open networking protocols, including http/telnet
- FlightGear has a built-in browser to inspect and modify internal variables directly
- i.e. there's no need to do any reverse engineering in FG
- FlightGear has a built-in scripting language that looks and works like JavaScript in your browser
- FlightGear has now a fully scriptable 2D rendering system that allows people to easily create custom instruments, custom HUDs, custom GUI widgets and custom MFD instruments.
I'm also interested in particular areas that FS-X is ahead in terms of features and usability. I'm aware that their terrain is better and there are lots of high quality aircraft.
FlightGear is especially interesting for people who already have some form of aviation or programming background. Most people without such a background, will probably find FSX/X-Plane much more enjoyable though - which comes down to usability issues. But also to lack of resources to get people easily started in an interactive fashion (tutorials, missions, adventures etc). Unlike FSX, we don't have an integrated "flight school" or other interactive features.
Also, anybody who's ever flown an aircraft in real life (PPL, CPL, ATPL), will probably agree that the interactive features in FS-X or X-Plane are much more appealing to real life pilots, i.e. to maintain proficiency (instrument check etc). I know people who are long-time FG users, but who prefer using X-Plane for these reasons, because FG is great if you want to help
developing a simulator, and if you are interested in programming and software engineering - but if you are only interested in actually
using a simulator to get prepared for your checkride, there are unfortunately still today much better options. Which doesn't only boil down to usability, but also to certain features to actually evaluate a flight (i.e. flight path profile) or having a virtual instructor.
One of the greatest issues we are still facing is the large number of aircraft that are nowhere near completion to actually model important aircraft systems to provide for a realistic flight experience, including systems/failure modeling. While changing that is obviously a huge effort for complex airliners, it should actually be do-able for smaller aircraft. The c172p and the seneca are pretty good FG examples actually.
Obviously, real life pilots are unlikely to take FG seriously as long as we have 300+ aircraft of which 90% have no realistic systems modeling or bad flight characteristics, that don't meet their real life experiences.
And then, it's still about providing a real value to end users, so that they don't just need to use their own imagination to define a purpose and use - i.e. obviously using FG to shoot approaches is just as well possible as it is in FSX/X-Plane, but in the latter case, there's real support to make this swift and easy (and much more actually), i.e. there are dedicated "flight training" features to help with proficiency related tasks.
These features are there for people who have used simulators before, but also for people who are completely new to simulators - similarly, you can actually use FS-X to learn about flying (interactively!).
I'd reckon that most FG users have previously used some other flight simulator before.
Not being able to easily change aircraft or reset the simulator properly is obviously another showstopper in comparison to FSX/X-Plane, because there, redoing a segment of a flight, or saving/loading a flight, is simply possible - which it isn't in FG. Which makes such use-cases unnecessarily tedious.
If we really want to attract more FSX/XP users to present FG as a free and viable alternative, we would first of all need to make sure that we are serious about what we do here, which inevitably means focussing more on "real life" features, features that appeal to people with a real background in aviation, not just from the PPL/CPL perspective, but also from the instructor perspective. At the moment, FG is of really little use here unfortunately.
Personally, I'm attracted to FG because of the ability to modify it, inspect/change internals and contribute to it, not because it's such a great flight simulator or because of its instructional value for real life pilots. For such purposes, people tend to find FlightGear's commercial counterparts much more appealing, and really don't mind having to pay money for it.
It's a little bit like electric cars: they are all neat and dandy, but with a range of just 50 miles they're of little use obviously, and more suitable for hobby use ...
When the git project recognized that they had usability issues, they tackled it in a cool way and started conducting yearly end-user surveys via survs.com to adjust their development priorities accordingly:
https://www.survs.com