Whatever you think about Bob Brown, there is no question he has been an incredibly successful politician, one who has grown support for the Greens Party to record levels as well as formed a minority government and ushered in the introduction of a price on carbon.
Christine Milne, having been the Greens chief spokesperson on climate change issues for some time, already carries a high profile.
From what has been seen publicly it seems unlikely that her leadership will signal a wide departure from what the Greens have stood for to date. Also with Milne what you see is what you get. She behaves the same way behind closed doors as in public.
What may be differ most is the personal style and communication ability of the two. Milne tends to come across as a lecturing mother, whereas Bob Brown is a little softer and possibly more charming in demeanor. Still, this may not be an electoral hindrance in appealing to those people with a predilection towards voting for the Greens.
In some respects these voters may like Christine Milne lecturing us about our short-sighted and selfish behaviour. But this lecturing style is probably unlikely to help broaden the Greens electoral support beyond the already converted.
Also it may make some in the business community and the working class more hostile and suspicious of policies that the Greens have had some influence over.