Plenty of new gameplay ranging from small details to huge additions, but needs ironing out that patches haven't fixed.
The three primary features advertised in LN are the addition of several types of bars (where your Sims can hang out, eat, dance, enjoy a hot tub, or bartend), celebrities (which your Sims can become themselves for quite a few perks, if you're willing to deal with the downsides), and of course, vampires (which are abundant in the town of Bridgeport). The bars are great additions; they're a worthwhile place to go to meet people and run into celebrities or vampires and just generally make getting out of the house feel much more worthwhile. Celebrities add a great new dimension to the game, although not necessarily in a very realistic way. Your agent will call all the time with promotional opportunities (which pay extremely well!) to help your Sim earn celebrity points and become more famous, but really, the only practical way to become more famous is to befriend celebrities. It would have been much more fulfilling if actually accomplishing worthwhile things (for example, writing a best-selling novel, painting a masterpiece, reaching the tops of careers, etc) contributed, but these have absolutely no effect, which is disappointing. It's all about who you know, and nothing about who you are. The vampires are a kind of neat addition to the game as well, except for their incredible abundance in the game. In Bridgeport, it is not even a little bit challenging to find a vampire and then befriend them enough to ask them to turn your sim. On the bright side, being a vampire has many perks, arguably more pros than cons, so not everyone will find their quantities disappointing.
There are several other game aspects that are noteworthy beyond the well-advertised ones. First of all, there's a subway system in Bridgeport (and you could easily add one to your pre-existing towns in Edit Town). Walking into one tunnel transports you to another, and choosing "Play for Tips" is insanely more profitable in a subway than in the park (you can earn thousands in a few hours if you have a high enough skill). You can't actually see the subway, which might disappoint some, but the instantaneous transportation aspect certainly makes up for that. You also can't see any other floors or rooms in high-rises (including apartments) other than the ones that are built on. This helps the game run much more smoothly, I'm sure, and I personally don't mind at all, although some reviews have complained. You still have neighbors in your apartment building; they're just not placed by you.
LN also adds the ability to join the film career, or even more notably, to create a band. It's easy enough to create a band, but just about impossible to actually succeed with it. After creating one by simply inviting members, you have to wait for a phone call from your agent to get a gig (that's right, it's an opportunity). This is a huge problem. First of all, it can take weeks to get a single gig, and weeks more to get another. Secondly, all your band members need to show up at the appropriate time, and if you don't have them all in the same household, this proves problematic. The game supposedly automatically sends them to the lot where the gig is, and you can always call them if they still don't show, but that doesn't mean they can't flat-out cancel on you. Long story short, don't even bother making a band if you aren't controlling all of the members. Even then, you're likely to struggle way more than feels necessary. Hopefully in the future a patch will make gigs easier to get, or even make it possible to request them instead of waiting for them. There are several other apparent glitches that patches have not yet addressed, and while I wouldn't normally address this in a review only months after a game's release, the number of glitches seems abnormally high, not to mention the fact that there are quite a few World Adventures and Ambitions glitches that have yet to be rectified. It's definitely plausible that EA won't bother to remedy these new glitches, either.
In spite of the hugely disappointing failure of the new ability to create a band, Late Night still stands as a very solid expansion pack. It adds plenty of new gameplay, almost all of which is far more substantial than what Ambitions had to offer, and integrates much more easily into the existing game than World Adventures did. While the other two expansion packs definitely have their perks, Late Night is the most well-rounded expansion yet.