This is a direct copy and paste from a Readme that will accompany the release, and gives a nice overview of the cool things this program is gonna be able to do.
The original version made an attempt to write the lit data into the same filespace as the original data, but was not always successful, leading to corrupted lightmaps on a few surfs. This one is successful (I can't say "always", but the ID1 maps I've tested work fine).
I've replaced the old DOS UI with a proper Windows UI. It looks nice. No more messing with paths, or trying to make sure that you've got filenames right; just double-click the icon and browse for your maps. It even makes a (kinda) reasonable stab at guessing where you have Quake installed. Welcome to the 21st century!
Multiple file support
Think of it as a kind of batch processing mode. If you want to light multiple BSPs at once you can now do so.
Large BSP Support
All map limits have been removed. As a bonus, we're now a *LOT* more memory-efficient with regular size BSPs that ID light was.
Custom Palette Support
Fullbright colours in the Quake palette are used for helping to determine light colours (a light close by a texture which has fullbrights in it will pick up it's colours from those fullbrights). Previous versions used a hard coded palette in the executable, but no more! You can now use this with mods that have a custom palette. It will even pick up a palette from a PAK file, and searches relative to the location of the current BSP it's working on. If it can't find a palette on disk it falls back on it's own internal copy of the Quake palette.
4 x Oversampling
This can give better results (although at the limited resolution of Quake lightmaps there's only so good you can get), but unfortunately it doesn't work so good with pure LIT generation, so it's only available as an option if you choose to also regenerate the light data in the BSP.
Lights BSPs in PAK Files
You can open a PAK file, browse the BSPs in it, and select to generate LITs for them. If you have selected to replace the light in the BSP it will work slightly differently (obviously), and will instead write the BSP out to your /maps directory (rather than stomping over the data in the PAK).
Right now I'm thinking that I'm going to miss my Tuesday deadline; all was well until I started adding in PAK file support, but I'm of the opinion that it's an important enough feature to justify the delay.
Another UI change might happen - I'm liking the idea of an Explorer-like tree view incorporating BSPs from both the file system and from PAK files.