All the things on your list are possible.
I have seen custom combat feats myself so I know that for a fact (Lunge, Low Blow etc.). You can also modify the sneak attack range and weapon range. I have not heard of any of these things in NWN1 so I assume they are new. [Edit] Also charge and shield bashing.
Area instancing is fully supported. A couple of persistent worlds use it extensively, see Trinity for example.
Jumping, you can use the RWS levitation animations. There might also be a jumping package. I don't know. You could conceivably do some calculations on the Z axis to determine if a player would make it from his original location or not, if its too far up / or away.
You can make as many custom animations as you want if you know how to do it. I've never looked at it but there are plugins on the vault for using Gmax I think. Including recent editions of that program. But I leave the details of this to people with more knowledge about it.
Pushing sliding like in the Baldur's Gate games (dragons) is fully scriptable. I've seen it done well in several modules and persistent worlds. Kamal also made a sliding mini-game, where you slide down a mountain / hill.
I don't know exactly what you mean by swift gameplay and posession.
You can make creatures possessable by adding them to the party, if that's what you mean. NWN1 didn't support parties, so that's new in NWN2. There is a script function to mark creatures as not possesable / possesable.
Swift gameplay, I've never had issues with big exterior maps. I would answer that it depends on the competence of whoever makes the exterior area. Because if you add a lot of stuff that isn't needed then it's going to take a toll. But that's also the case with NWN1 areas if you use trillions of placeables. So I imagine that's easy to get a grip on for people who developed for NWN1.
There aren't any issues with PvP, though I'd recommend using the KEMO suite or your own GUI code to tailor it to your world. Like you can modify the context menu so that it sets people hostile more easily. Or you can do it via factions / battlefields, or arena type PvP.
There are also tricks you can use to support many more NPCs in your battles without causing lag, so you could have pretty big battles I think.