I had a theory that since McCullum started opening again in ODIs, and especially since Moles began coaching, he scores a LOT quicker when chasing a target, then when he is batting first.
I went to statsguru, and was proved correct:
Since late 2007, when he started opening again:
Batting first: Ave 34.10, S/R 79.02
Batting second: Ave 42.90, S/R 107.92
Huge difference there, not just in strike-rate, but average also. Which gives some weight to posters saying that he is more successful when he has an attacking mindset.
Since Andy Moles became coach (admittedly a small sample):
Batting first: Ave 32.85, S/R 65.52
Batting second: Ave 25.00, S/R 85.93
HUGE difference in strike-rate there. A strike-rate of 65 opening the innings is unacceptable for any type of batsmen these days.
Both samples are effected by that one innings where he batted injured at no.9. But it's only one innings, so it shouldn't affect the averages too much.
I feel that when batting first, he places responsibility on himself but sometimes digs himself (and the team) into a hole by doing so.
When batting second, he either thinks, "I'm chasing 300, our only chance here is to go after the bowling" or "It's a moderate or low target, I can ease the pressure on the rest of the batting order's required run-rate by going hard in the early overs."