Networking: Most folks building a bespoke networking layer seem to go with ENET, which is cross-platform.
Async file I/O: yup.
Threading libraries: something wrong with Boost Thread? (Boost gets coroutines, too, in 1.53, which I'm pretty excited about.)
Honestly, the biggest problem I see in writing a cross-platform game is device input. Trying to build a joystick/gamepad-based game for Linux and OS X are pretty painful. Fortunately I've lucked onto GLFW as a windowing toolkit, which handles the HID Manager boilerplate and other badness for them.
It also supports Android, so in theory you could almost have one code base with only a UI and input revamp for Android target every PC os.
Also, SDL 1.x is LGPL (instant dealbreaker, I build with static libraries on a Mac to avoid dylib hell) and SDL 2.x isn't done. So there's that, too.
In the spirit of abstraction and not having to build the object model I'd get for free with Direct3D, I used SFML for a while; the general idea of it is promising, but the abstraction is janky, it's desktop-only, and I still have a bitter taste in my mouth from the way they handled showstopper bugs in SFML 1.x as far back as three years ago: "Completely reproducible crashes on all modern ATI cards? Just upgrade to our incompatible 2.x alpha!". (2.0 still isn't out, which makes it super groovy.)
.
I now use GLFW for windowing/input, ENET for networking, and PhysFS for I/O; I can pretty easily use each of them in a way that maps pretty well to my own preferences for an object model and a mental model. Probably not scalable (for a multi-person project or a 3D game I'd almost certainly use OGRE rather than doing it myself), but nice for my purposes.
The only part of my codebase that isn't pretty trivially platform-independent right now, without any special effort, is in "platform.cpp", which does the tremendously difficult work of finding me the user directory and the appdata directory. =) I sure haven't done anything special to make it build both in Xcode (my primary environment) or Visual Studio.