I can only speak for Auckland.
Clubbing is pretty good, although during the winter it's usually shit because it rains a lot and there's less of a turnout in town. There's "big name" overseas DJ's coming over every few weeks, and usually once every year or so per DJ. I wouldn't rate it as good as Sydney, but the Auckland clubs are all within walking/$5 taxi distance of each other.
The amount of money you need to make will depend on where you're living, and what you need to pay for commuting to/from work. You can easily afford to live in a highrise studio in the middle of Auckland city if you're earning $30k/yr plus (that's probably the comfortable minimum).
Finding work is pretty easy, but it depends on who you know, so you'll find it easier if you know someone who can hook you up with an interview. If you know what you're talking about, and have some experience, you can usually find a job within a week or two of searching. The basic "Helpdesk" job pays the least (most places starting at around $27.5k/yr), but it's really easy to get to $50k/yr within a short period of time if you know what you're doing.
A good website to start looking for jobs is
http://seek.co.nz/
The beaches around the city are kind of shitty (if you're into surf beaches), but it's a 30-40 minute drive to beaches like Piha/Muriwai/KareKare/Bethels beach which have some of the best "public" surf in New Zealand. I wouldn't recommend learning to surf at those beaches though, as there's undertows/rips, and 4-6 foot swells on average.
Weather is kind of weird. It can be hailing in the summer, and 27C w/ no clouds in the sky during the middle of winter. Or you can have all 4 seasons in one day.
Skiing in the North Island isn't as good as the south (for obvious reasons), but the ski season is usually around 7-9 months long, and Ruapehu ski fields are just a 3-4 hour drive out of Auckland.
The women are questionable, many are fucked in the head/like playing mind games, and often have baggage, be it problems, or kids. Although it all depends on who/where you go to meet them, as I'm just generalizing. However I've probably been "spoiled" by australian chicks because I grew up in Sydney. The coolest chicks are probably the brazilian + european chicks that hang out in Downtown auckland, near all the backpacker places.
Kiwi's are usually indifferent regarding Americans, and if you're "cool" they'll like you regardless of what country/religion/ethnicity you are/from. However the loudmouth "tourist" americans that whine about everything
really piss people off. But I'm sure that's the case all around the world.
Traffic is shit, so you've got a few options.
A) Find an apartment/job in the city, and walk/bus 5-15 minutes to work.
B) Find a cheap house in a suburb outside the central city/job in the city, and expect to spend about 1 hour commuting on what would normally be a 5-15 minute drive in no traffic.
C) Find a place/job outside "central" and you're usually fine in regards to traffic, as long as it's not rush hour.
The train service is the equivalent of 3rd world, but you can catch a bus to pretty much anywhere, at any time of the day. (which is the complete opposite of Sydney)
It's not like "bigger" cities where you worry about getting mugged if you turn down the wrong alley. You can walk past a group of 20 drunk islanders (stereotyping) outside their favorite hiphop club and they'll usually be like "Hey bro, hows your night going? Score any pussy tonight? Churr laters". Of course there's exceptions to this, but if you know a martial art I guarantee you'll be sweet (as there's no guns in public).
The government is full of morons (like this maori guy who got given a few million dollars for some reason and spent it on buying boxers/underpants made out of gold), but that's the same for most countries I guess.
The best thing about the city, is that you're practically a short drive from everything.
I'm 22, and have lived here since I was 12 (FYI if you're wondering).