Trains a mix of antiques (notably the ~50 year old C trains) and new modern stock and plenty in between. Much bigger feeling inside, air conditioned, generally clean but many rather run down. Good electronic PIS on the new stock, manual announcements on many. Plenty of variety.
You'll notice the trains and lines are much much less smooth running than in London, which really hit me last time I was in London.
24/7 operation is great but be prepared to wait up to 20 minutes evenings and weekends and beware some lines (eg B) do not operate during these hours.
Engineering works can be unpredictable and inconvenient at weekends.
Beware of lines that run fast - it's quite easy to work out from the subway map though.
The unlimited MetroCard is great value, though the standard $2.50 flat fare penalises those travelling only a couple of stops. If you're here a week the 7 day card is very good value.
Stations much more spacious but in much poorer state of repair and cleanliness. Tramps - often mentally ill - are very common in stations and trains (particularly in very cold temperatures of January) and though you don't feel threatened, they're a nuisance and are the cause of empty coaches or parts of coaches on packed trains due to the odour. I've occasionally felt uncomfortable due to the more unstable characters, though this is unusual and the system is very safe, within most of its extremities.
The barriers often are temperamental and very outdated.
Lots of entertainers, singers, groups, dancers which can be entertaining but can be extremely intrusive and irritating if not in the mood.
Passengers can be very inconsiderate, particularly boarding before you've alighted and through noise and personal space.
Finally, a strong recommendation for an afternoon out... the New York City Transit Museum at the former Court St station in Brooklyn is a must for a transport fan.