More Rewire audio channels
As far as I understand it Reason 4 supports only 64 channels (that means 32 stereo tracks) audio out via rewire.
This sounds quite much, but if you rewire a drumkit with several stereo channels into another application, e.g. logic, you run out of audio channels quite fast.
Rewire 2 features 256 audio channels. However, it seems that Reason 4 is still on rewire 1 status with 64 channels of audio.
256 channels may be overkill, but 128 channels - 64 stereo channels - would be a huge plus.
Another thing: Most people route stereo channels into other applications. Labels with L or R on the back of the hardware interface would be nice. Since Left channels are the upper row and right channels are the plugs on the lower row if you route stereo - why don't write it down.