The one feature that I find to be a glaring omission in Reason 6 is the ability to link multiple channels together on the mixer so that their levels can all be easily adjusted at one time. This is a core feature in workstations such as Logic, Cubase, Reaper, etc., where selecting multiple channels allows the user to adjust their levels such that amplitude will scale to keep the relative channel levels consistent.
I teach pro audio production and a very significant topic in our lessons is gain structure. A common problem that occurs in mixing is that the sum of a number of the mixer channels begins to cause output clipping. A simple remedy for this problem is to link the offending faders together and attenuate their level. It is very common during the mixing process to periodically link all of the faders on the mixer together in order to reduce their level and create more headroom for the mix.
Currently the only way to resolve this problem is through complex and time-consuming workarounds which involve manually routing CV to create fader groups:
Mixer channel fader linking would be a very simple feature to implement and it would really enhance Reason's efficacy as a professional mixing environment and teaching tool.