Congrats to the admin on what must have been a heck of a hard job.
Even the avatars and signatures transferred. This was big surgery and seems to have been flawless.
The good: in many ways clearer, works better from mobile, and above all accepts attachments so posting images will no longer be painful.
And as Ries said, video seems to work great too.
The bad: fonts need some work.
The site prefers Segoe UI, if absent (on any mobile device or non-Windows desktop) defaults to Helvetica Neue or Helvetica.
The difference in fonts is quite dramatic, which doesn't make for a consistent experience across devices.
I transplanted Segoe UI from my Windows machine to my Mac (a move of questionable legality), and things look better now.
With Helvetica Neue/Helvetica, it doesn't look very neat. Lots of different sizes all kinda running into each other.
Some work to provide visual consistency across devices will be great...
Most Many people don't care about fonts. But those who do, really do.
Edited to add GIF that shows this really well.