Apart from the usual graphics and performance frustrations, my main gripe goes to the lack of expandability of the ESP engine.
1. Custom ground polygons must be written using FS2002-style SCASM code.
2. There is no way to recreate switch and lever sounds without using a custom .dll gauge.
3. Turboprop engine start timing sequences can't be modified because the start animation is built into the .mdl file.
4. Again, in turboprops, the propeller doesn't start turning until combustion starts, which is unrealistic, especially for a free turbine like the PT6.
5. Flight dynamics have limited functionality and proper spins are very hard to come by.
6. Default GPS nav data is set to what it looked like in 2005-2006 and it's locked there. There is no way (as far as I know) that a user or 3rd party dev could completely overhaul the sim's native navaid, waypoint, airway, SID/STAR and approach databases.
7. Stock airports cannot be deleted or given a different ICAO code.
8. Airports must lie on an airport flatten polygon unless the dev is willing to spend time modelling each undulation of the surfaces.
Don't get me wrong, I love flight sim (I'm using P3D3 right now). However, compared to other current game technologies (just look at how the cityscape looks like at sunset in GTA5, and all this presented to you in 60+fps!), flight sim tech leaves a lot to be desired.