I think the resignation was more a matter of Brash having a bit too much egg on his face after one too many gaffes to be able to continue credibly.
As for repercussions – the man likely to replace him, John Key, is rather younger and more energetic, by all accounts, and so may make a stronger Leader of the Opposition, and perhaps a stronger challenger in 2008. Beyond that, I admit I know little about him other than what’s in the newspapers this morning…
Our political parties in power tend to go through cycles here. With great public support in opposition, because they’re not the ones making the rules and can promise to do a heap of things for voters, they’re swept to power (these days, under MMP, it’s not so much a landslide as used to happen, but we get the near-equivalent). Then, in power, there’s usually a good 3-6 years. After that, criticism grows, discontent rises, the opposition gets cocky, and then they get elected back in.
The usual lather-rinse-repeat routine.
National here still stands a good chance of winning the next election. Brash himself was a new kid on the block, essentially, and they nearly swung the last one. A fresh leader won’t make all that much difference to the cycle. It depends on how much of the dirt dragged up by Nicky Hagar’s book sticks to National as a whole in the voters’ minds, or whether they can simply scapegoat Brash and move on.