I put about 15 hours in this weekend.
I can't say I've seen a lot of the new stuff, because importing my save file seems to have jacked up my wins-per-character.
Pre-Afterbirth, I knew I had beaten both
and The Lamb with most of the characters. I import my save file and many of those wins are not recorded (even though the achievements and unlocks for the items were clearly still unlocked.)
So I spent most of my time replaying to get the first 5 wins with most of my characters, again. In order to see the new end levels, I think you MUST beat Mega Satan, which is easier said than done as you're not guaranteed two angel rooms per run. So I basically took 15 or 20 hours to rewin all the old game content while not seeing much of the new. That said...
Closets are now a thing and they fucking suck. If you think fighting your least favorite enemy in a room about 1/3rd the size of a normal BoI room is bad, imagine what fighting DOUBLE TROUBLE for your least favorite boss is like under the same circumstances.
There are rooms now which are literal traps. You walk in, door shuts, turrets start firing and you have to hit one or more switches in the room for the turret/spikes to turn off and the doors to open. It's basically "Hey, I heard you like health. Let me tax you some of it then." While many of the rooms aren't bad, some are basically unpassable without using some health somewhere unless you've got flight.
Occasionally the item in the treasure room on the first floor is surrounded by rocks and a tile of spikes. Because you know what's fun? Missing the first fucking treasure room because you can't take the damage or have any bombs. THANKS ED YOU FUCK.
The Scarred Womb is probably my favorite new alt-level. If you thought Womb was gross and/or evocative......Scarred Womb is that x4. From the music to the visuals, it leaves an impression on you.
Really enjoying the music for the expansion.
Many of the new pills are worthless. They're just weird effects like Retrovision which have next to no impact on gameplay. Normally I wouldn't care but all the new pills just further dilute the pool of good pills with shit ones. Even with Cain + Extra Luck + PhD, the bulk of pills I run across aren't worth it. That combo typically made a run in previous versions.
Some of the new bosses are a little over the top. Rag Man, Little Horn are easy (Little Horn spawns every fucking game right now because he's Ed's new favorite but at least he's easy.) But there's one that basically throws out Mom's Heart levels of bullet spam and you can get him the first level. I believe he's called the Stain, modeled after that one Womb mob that sinks into the ground then reappears. It's kinda bullshit to throw that at the players before they've even gotten anything.