There are some bugs in the current GI system which cause unusually dark shadows in certain situations, for example when using Fake Stones. A global "shadow lightness" control is somewhat against the idea of Global Illumination when properly implemented, but it may be something we can support for convenience in the future. Certainly we will add additional control to the greatest degree reasonably possible.
In the meantime you can balance enviro light "strength on surfaces" and sunlight, use additional suns, etc. as others have suggested. There are a few workarounds, but I certainly agree they're not ideal from an ease of use and artistic standpoint. Another possibility would be to save an HDRI image as .exr and adjust it in Photoshop using levels to isolate the shadows.
I've also gone ahead and posted the Fill Light setup to the Shared Files area. I hope you find it useful. Instructions for use are in the post here:
http://forums.planetside.co.uk/index.php?topic=580.0C-A, I'm not sure what you would prefer us to be saying in regards to unimplemented features. We do our best to be clear as to what will be in the final product and that's really all the information that we can communicate at this point. I don't really see how the "fix it in post" analogy is relevant.
TG2's GI solution already supports color bleeding between surfaces. It's fairly subtle, as it is in the real world, and will often require high GI settings to be seen. But it is definitely in place and functioning at this time.
"Sample Resolution" is already handled by the existing GI detail settings, although perhaps differently than other solutions. I don't think you would complain that another GI renderer didn't have a "GI sample quality" setting though, so don't expect to always see a mirroring of familiar functions and settings in TG2. Solutions are implemented in the way that makes the most sense for TG2's specific needs and these often differ from more generalized systems in other applications.
We do intend to implement soft shadows in the future; it's definitely a noticeably missing feature in the current version. If you notice other problems or errors in the current rendering of shadows, please report them.
Gradient, Light sources need extremely high strengths at anything but the default size because the strength is proportional to light source size. Light sources do have falloff though, unlike the sun, so they'd be the preferred approach, especially in scenes with additional planets - a light source would not affect a far off planet.
- Oshyan