When writing my scripts in C#, what features do I need to avoid if I want my game to be compatible with Apple devices?
You don’t need to avoid anything at all. Unity scripts are compiled and the target platform doesn’t care if they were originally written in c#, boo, or unityscript.
There is no need to worry about C# compatibility with apple devices, or any devices for that matter. Unity can export to nearly any platform, and whether you use UnityScript, Boo, or C#, the outcome will be almost identical, and the API unaffected. The only thing I will say is that you should definitely stick to one or the other, as it becomes very difficult to reference C# scripts from JS/UnityScript because of the fact that C# is rendered first.