I don't know what it is - maybe that we're on the third series, and the third incarnation of the general theme of 'night out' with The Sims, but it seems like there's a growing sense of dissatisfaction with the game as a whole that is only exacerbated by each new expansion. That's not to say there still aren't hours of gameplay to be had with each expansion, but I'll be the first to admit I'm a little tired of the 'Hot Date' / 'Nightlife' / 'Late Night' theme that this pack is touting.
Of course there are some variations on the theme. For one, downtown is no longer a separate destination from the rest of your base city. The new town that comes with the game, Bridgeport, is jam-packed with high-rise apartments, dance clubs, dive bars, and cocktail lounges, plus many of the other venues have been combined to make for a more condensed, urban experience. If you're not ready to move onto a new town, you can also just snap up some pre-built buildings and place them in your existing town. In general Bridgeport is a cool place and all of the bars are pretty neat, each with their own vibe, although in general I think the dives and the dance clubs are the most fun. The high rises are different than what existed in TS2: Apartment Life in that your sim basically lives in a predefined space within an otherwise 'empty' apartment building. There are no longer multiple apartments on a single lot to manage, just your sim's place and their neighbours, who they can attempt to meet and befriend. In short, it's basically like living in a home in the sky.
'Late Night' also incorporates the 'celebrity' aspect of TS1 'Superstar' expansion pack. Your sim can work their way up the celebrity food chain mostly by befriending or dating celebs, which earns them discounts and freebies, public notoriety, and lurking paparazzo and scandals around every corner. Truthfully being a celebrity gets to be a bit tiresome, and the way you work your way up isn't nearly as fun as it was with 'Superstar', but the freebies and discounts are nice, as is the ability to breeze into any bar you want instead of having to waste an evening trying to get into one when your celeb level isn't high enough.
The non-Sim creature introduced this time around is the return of Vampires, which are by far the best 'creature' of the TS3 series and superior to the vampires of TS2. There are quite a few vampires lurking through Bridgeport and getting turned into one isn't much of a challenge at all. Once you're a vamp, they take on a Twilight-esque pale, phosphorescent appearance complete with shimmery eyes. Unlike being a mummy as in World Adventures, being a vampire and carrying on a normal life is pretty easy. Your sim can't really spend much time outdoors during the day, and they drink blood (either from the fridge or from a willing human friend), but other than that they can have jobs and babies, gain skills super fast, have some mind control abilities, and have the ability to run like the wind through town.
Beyond that, there is one new semi-traditional career, similar to the expanded doctor career that came about with Ambitions - acting. Your sim can also form a band and play gigs around town, learn the fine art of bartending and moonlight as a mixologist, or simply live off the spoils of being a celebrity through the various opportunities that come their way. There are tons of nice new clothing items for young adult and adult men and women, mostly themed around party clothing (including formal wear, as some bars require your bar to change into their formal gear). The new furniture is generally pretty modern to suit placement in the various bars around town, and the best new items are probably the professional bars (for learning mixology) or the various new instruments - bass, drums, and piano. It's tons of fun to have a jam session with four seasoned musicians, for the record. Hot tubs and custom-shaped pools are also welcome new additions in the build / buy modes.
Overall Late Night adds just enough that I can say I've been content to spend countless hours with the game since its release. It's nice that in comparison to the first two expansions, this is an expansion you can just sort of play and not have to shake up your current sims' entire life and career choices. Fans will be happy that not all venues are rabbit holes as they were previously, and there are enough new career options to explore that if you're stuck on playing with one family, you could easily spend a few generations getting to know the full expanse of the game. I feel like the game sort of offered up portions of past games - both from TS1, TS2, and TS3 - for a mix of different things that will appeal to various players - new skills, new career paths, new adventures, new design objects, and a new look and feel for towns. The result is a game that isn't particularly strong in any one area (as World Adventures was with the introduction of questing and Ambitions with 'professions'), but a decent expansion for your day to day gameplay.