In the world of late but informative, Peeve has released a video regarding mechanics based on a reddit question list. Some of it has been covered previously, some information is new, some is correcting previous information (weapon arts DO replenish at bonfires). There's also a clip of him fighting the boss @ 39:50.
https://www.youtube.com/watch?v=tud0kKjwwvM
+ Roll out of gestures YES
+ Walking with gestures NO (yet)
+ Drink estus on ladders YES (and pop souls)
+ Take a step or two at the start of estus drinking animation YES
+ Ravioli step NO
+ Dead angles NO (in PVE at this stage)
+ Toggle-escape NO
+ Animation cancel NO
+ Stored roll YES (queue up rolls by tapping B while rolling, which lets you immediately roll again)
+ Riposte weapon swaps YES
+ Parries with large weapons in twohand stance (ex. greatsword) NO
+ Weapon sheaths NO
+ Riposte breaks lock-on YES
+ Layered weapon sound effects YES (Bloodborne when you press R1 R1 R1 each R1 has a different sound)
+ Counter damage UNKNOWN (could not tell/just bad playing probably)
+ Positional hit damage UNKNOWN (ex. greatsword tip vs base no difference, need a halberd to test)
+ HUD in the demo is a placeholder
+ Could not determine anything about poise or stats
+ Hyperarmor active in the greatsword Lunge weapon art
+ Hyperarmor (but not iframes) on backsteps, makes a distinct sound like a weapon brushing off metal
+ Forward+R2 does regular R2, you need to sprint+R2 OR backstep+R2 to do a jumping R2
+ Does not think backstab cone is SEVERELY reduced as some others have said, if DS1 cone is 60, DS3 cone is 40
+ DreadedCone mentions doing a regular hit or pre-hit before backstab, but Peeve thinks it was DS1-style instant
+ DS1 parries
+ DS1 instant estus
+ DS1 combat/movement, very fluid, 1.3 to 1.5x DS1 movement speed
+ Faster greatsword R1s
+ Exhausting stamina does not affect swing speed
+ Legion scimitars: twohand stance is holding both scimitars, not holding one scimitar with two hands
+ If you buff one scimitar, then switch to twohand stance, both scimitars will be buffed
+ No stamina regeneration if you delay second swing in a combo
+ Both demo characters could roll 5x without stopping (endurance not known)
+ DreadedCone said weapon arts do not replenish at bonfires, Peeve says yes they do
+ "DS1 in Bloodborne-detailed universe"
+ Detailed combat animations, no lying on back in air and then sliding on back after golfswings ala DS2
+ Tried to backstab red cape knight by locking on and circling around him, red cape knight reached over his shoulder and performed a backwards shield bash to punish him
+ Phone recording of part of Peeve's first fight with Dancer boss @ 39:50