One small section here for specific notes on driving in New Zealand, since it's a very rural country and you need to do a lot of it to see anything. They drive on the left (the correct) side of the road, as every rental car agent kept reminding me (I should never have shown them a US driving license!).
That's not that hard to get used to, though, and automatic transmission seems to be plentiful for those who can't drive manual. What is very different is the type of road network - New Zealand is so sparse that most roads are one lane in each direction on the same piece of tarmac (a single carriageway road), and often winding and hilly.
Don't expect to get anywhere fast, and don't expect to be able to overtake a slower car until you get to a nice straight section of road - which can take a while. Similarly, faster cars may come up behind you and sit behind, looking for an opening; just stay calm, and if there's an easy pull-out, take it and let them past. I didn't encounter many aggressive drivers, though there are some. There are also occasional single-lane bridges or road sections; just have a good look and follow the signs, and you'll be fine.
The road conditions are also a little less than perfect, as some of the more rural roads are not fully sealed, and there's often small rocks and gravel lying around. Expect to get small rocks dinging your windscreen, and make sure you have coverage for it; I got a nice big chip out of my windscreen from a passing lorry near Christchurch kicking a small rock towards me.
There's also plenty of pure dirt roads, fords, and similar road features on the even less travelled roads, like those out to trailheads; I'd recommend having some experience on this terrain before you try driving it yourself. Most cities seemed to have guides that would happily drive you to trailheads and walk the trail with you if you didn't want to do it yourself; remember that there's often not great mobile phone signal, too, so calling for help might take a while (or mean flagging down a car to get a ride to where there is signal).
That said, if you've driven anywhere rural before, it's not hard at all - it's just pretty much all rural, apart from a small section around Auckland that's an LA road planner's dream (and has the traffic to match).
My final tip is, if you're planning a one-way rental, try to go south-to-north; most people drive north-to-south, and you can often get much better rates (or sometimes free rental entirely) going against the flow.
Ka kite ano, Aotearoa
Usually, I go somewhere and come back with good memories and inspiration of where to travel to next. With New Zealand, I instead came back with an even longer list of things to do there than I went with, and a small sense of loss, like I'd lived somewhere for a while and had to move away.
It's one of the best trips I've done in a long time - not that I expected to dislike New Zealand, as two of my favourite things are mountains and snow, but there's a real sense of place that comes along with it, and the welcome that was extended to me by everyone I met was incredibly heartwarming. The landscape is so varied, yet all of it has a story to tell and something unique you'll find nowhere else.
For most people, New Zealand is a long way away - but that is, in my eyes, part of its attraction. Humans - and, apart from one species of bat, mammals - have only been here for a millenium, resulting in an ecosystem that's gone in some spectacularly weird directions. Combined with the volcanic and tectonic activity shifting and reshaping the landscape, and you end up with something truly special.
I'm not sure when I'll go back - part of me wants to go while I still live (relatively) close on America's west coast, part of me would even happily move there - but I will almost certainly be back. And maybe, if I can arrange it, with a small plane and a pilot license validation.