There's not a whole lot of new material, but what is new is pretty cool.
Some of the new items include a karaoke machine, a DJ Station, and a Dance Sphere. With the Karaoke Machine you can sing a Solo, or Duet with that Special Someone. The DJ Station allows you to act as a DJ, and best of all, you get paid! With the Dance Sphere you can show off to your date how you can stay on this crazy machine and the whole time you can raise your fitness.
To most of you who don't yet have this game, a 'date' is inviting someone over to your house and attempting at a nice dinner. But with Nightlife you can skip over many of that. For one, you do kind of invite them over to your house at first, but the date is now graded, depending on if you give the lucky person what they want, no you can't go to the movies, but you can do nearly everything else. From taking them to a nightclub where you dance the night away, to taking them to a fancy restaurant with candlelight, that's not including the bowling alleys. No doubt there's much to do in this expansion.
I haven't yet mentioned the new 'species' of sim. Vampires are the new type of sims. Just like with pets we have Werewolves, Seasons we get Plant Sims, and University gets Zombies. Grand Vampires are the only way to make a Vampire w/o cheats. Grand Vampires can be found in the new Downtown area at night. To become a Vampire all you gotta do is raise your relationship with a Grand Vampire until they bite you. Once you become a Vampire your sim may want to get a cure to Vampirism that you can get from the lovely Matchmakers.
Matchmakers do exactly what you think they'd do. Depending on how much you pay them, they can get you someone you'll love, or someone you'll hate. They also sell Vampirocilin (A cure to Vampirism) and some cure to Werewolf-ism. They also give you a One-Time-Only potion that allows you to change your Turn-ons and Turn-offs.
Turn-ons and Turn-offs change who you'll be attracted to. Your turn-ons can be the craziest from Stink and Fatness to the smartest like Cologne and Fitness. Not un-like your Turn Offs.
Cologne is a new item the geniuses of Maxis invented and brought to the Sims 2. Cologne is bought at any store you can find that has a Cologne stand and you can be anywhere from one to ten. They each only have one use, and you can find them in your inventory. Your inventory is pretty handy, as well. You can take any item in a lot that you can go into buy mode with and you just click and drag into your nice little inventory, carrying it around with you until you need it.
I hope this has helped
-Jlybelly :-)
P.S. I didn't even mention the ownership of Cars and Taking a Group out on an Outing (which is graded like a date) or just for fun, both raising your relationship with everyone in the group.