It seems as if Nissan designed and programmed this new 7AT for fuel economy over performance, and that could generally explain why these issues are coming up. It seems to be the opposite of their 5AT which seemed to be designed and programmed for acceleration/performance over fuel economy. The 5AT has been well-regarded by most reviewers as one of the best and most sport/peformance-oriented automatic transmission ever made. This would also explain why there is barely any difference in 0-60 and 1/4 mile times between the 2008 G37 and the 2009 G37.
But I think the problem some people may have with the 5AT .vs. 7AT issue is that although the 7AT seems to be geared for maximum fuel economy, it only gets an overall 1-2 mpg improvement in fuel economy over the 5AT if all other factors remain equal. Not to say that 1-2 mpg improvement isn't a good thing because it definitely is a much-needed improvement, however, what you end up with is a 7AT that is reluctant to downshift and upshifts prematurely to get up to the highest gear possible (along with other reported issues), yet the fuel economy benefit is minimal.
I think that the torque converter being able to lock-up at such low speeds on the new 7AT could possibly be causing some of these issues. Although this does help with fuel efficiency and emissions I think that it might be making the transmission very reluctant to downshift even around town when passing-power is needed and is also making the downshifts feel rough and jerky, and could be making the transmission feel confused and overall less responsive when more power is called upon.