Most notably, the secondary rear camera packs a wider-aperture (f/1.7 versus the old f/2.6) secondary rear camera that should let in much more light. It's not being used for zoom, mind you -- rather, it combines pixels to take better low-light photos than you'd get with the main camera. The other cameras haven't changed, but this update still makes the 5T considerably more competitive if you're a shutterbug.
The rest of the hardware is largely similar. You're looking at a Snapdragon 835 processor, up to 8GB of RAM, 64GB or 128GB of storage and a Dash Charge feature that promises a day's worth of battery in half an hour. OnePlus' custom OxygenOS has received a facelift, though. In addition to tweaking the interface for the new screen, there's a face unlock option (albeit not as sophisticated as an infrared system like Apple's Face ID) and some behind-the-scenes improvements that should lead to faster updates.
Now for the big question: how much are you going to pay when the OnePlus 5T goes on sale November 21st? Unfortunately, there is a slight premium. The handset will start at $499 (£449) for a 64GB model with 6GB of RAM, or $20 more than the 'plain' 5. With that said, the recent surge of extra-expensive phones like the Galaxy Note 8 and iPhone X makes the 5T seem like a relative bargain. Yes, it's ultimately a higher-end version of an existing phone (the Oppo R11s), but you might not complain given that you're getting many high-end features for hundreds of dollars less than a typical flagship phone.