This DLC adds approximately another 2 hours of game play, as well as 6 more achievements, and is pretty much more of the same thing you experienced in the base game. The gore factor, however, has been ramped up. It's somewhat grosser than the base game, especially after you meet Eddie "The Groom" Gluskin.
In the base game, you were Miles, a journalist coming to investigate a lead of unethical wrongdoings and torture by the Murkoff Corporation that was emailed to you. Here, in the DLC, you are Waylon, the tech guy who sent the email. The DLC starts with Waylon sending the email, but actually ends the morning after Miles arrived to investigate.
Waylon gets caught sending the email, and is forcefully committed, as were some employees from the base game. That brings to mind a lyric from the Eagles song 'Hotel California' about how "you can check out any time you want, but you can never leave".
Almost all of the areas Waylon explores are different than the areas Miles explored. There will be a few places you quickly pass through that you recognize, though. You will also briefly run into Chris Walker, and even see cameos by the twins, Dr. Trager, and Father Martin.
The base game had 2 big enemies. The major one was Chris Walker, and the minor one was Dr. Trager. This DLC also has 2 big enemies. The major one is Eddie "The Groom" Gluskin, and the minor one is Frank "Kitchen Cannibal" Manera. Also, like in the base game, here you will have random variants sprinkled about, some hostile and some not.
I did find a few plot inconsistencies with dates/times, Murkoff rules/policies, and the condition of certain locations from the base game, but none of them affected the game play in any way. They are merely annoying to a purist, just like when things in movies either don't match or don't make sense.