WaveShop might fit your constraints. It's free and open-source, it's less complicated than Audacity but still has plenty of editing features, and it has a straightforward, uncluttered native Windows interface. It's also thoroughly documented, has 64-bit and portable versions, has good surround support, and is being actively developed. From the WaveShop website:
"WaveShop is a free, open-source audio editor for Windows XP/Vista/7. WaveShop is fast, lightweight, and bit-perfect, meaning samples aren't altered unless they need to be. Editing a portion of an audio file only affects that portion; the rest of the file is untouched. Blocks of audio can be cut and pasted without changing their contents at all. This is especially useful for patching a finished master without corrupting its dither. Waveshop's features include peak, RMS and spectral analysis, normalizing, fading, sample rate conversion, audio generation, and more, all with unlimited undo and comprehensive help."
Screenshots:
http://waveshop.sourceforge.net/screenshots.html
Features:
http://waveshop.sourceforge.net/features.html