So I downloaded the SVN code last night to see what it looked/played like
The basic info
version currently is 1.1.359 (last version was 1.1.324)
Not too many updates have happened to the code, but here's my take on the diffs between the last version and the current stuff in SVN.
pyogg - This looks like it was done to reduce the memory usage of using multi tracks. I did not have pyogg installed so I couldn't see what difference it made. It may take up less memory, but it may use more CPU.
Pause - when you hit esc during a song.
More streaming sound - Unsure if this is because of pyogg, or maybe a newer version of pygame.
High Priority Timer for background threads - Unsure if this really affects anything. the code seems to do it if it is full screen.
Libraries - These are folders. Folders are sorted before songs (probably a good idea). The graphic for this is a spinny cardboard box. Gives you a count of items in folder

Find - There's a find by name. I'm not sure if it includes subfolders or not
Volume Controls - Menu items for volume controls for lead, rhythm, song. These items are also available in game if you hit esc
New backgrounds - There's a 3d stage model background now. There are also people outlines that move with the notes (like the flames). Looks kind of like what someone wanted for the bar/party mod.

Hit counter - There's a hit and miss counter (a la stepmania).
Wavy long note holds - Looks similar to GH when you whammy (BUT IT IS NOT A WHAMMY FUNCTION. DO NOT THINK IT IS). Right now you can't control the wave, and it actually does not look very good. It looks ok when the screenshot happens, but when playing it's ugly.
Light effects - On the stage background there are some lights they flash with the beat (probably the most visual cool thing in this version).

Themable fret colors - change 5th fret to orange.
New song - Fasten Me by Scenery Channel....ho hum.
Performance - Performance seems to have gone down, but not by that much. Probably because of the background stage and lights and wavy thing. Compared to standard 1.1.324, the SVN version is about 5-10% slower. Compared to RF-mod with all optimizations turned on the SVN is 30-60% slower. With pyogg performance may be better (or it may be worse), but I don't know.
quick comparison for overlap with RF-mod.
Similar to RF-mod: Pause, Folders. fret colors, search
Similar to RF-mod 3.0: priority. lead/rhythm/song volume,
Not in RF-mod: PyOgg, new backgrounds, hit counter, lights, wavy hold, new song, slower performance
Not in SVN: Memory leak fixes (maybe pyogg will help here), song timer, GHII tracks, new note mesh, song sorting, preview.ogg, frets credit, 100% accuracy, alternate keys, 2 fret converter, disable various options for performance, HOPO, scrolling high scores
I found a few bugs, like the preview song does not stop when exiting menus.
Also, there are no changes or updates to the multiplayer code that already existed in 1.1.324
-Alex