NWN and PnP are two entirely different beasts and the two will never get much closer than they are now.
PnP was not made for 'balance'. The classes were not made to be, or ever be close, to 'balanced'. Because it was not intended for PvP to be a big thing. Classes in PnP are based ENTIRELY around what they bring to a PARTY of COOPERATIVE individuals. Not who can kick who's butt around the block in the most 'balanced' way.
In PnP one PC doesn't take on an entire encounter alone, let alone solo dungeons. (At least that's not the intent. It's a party-based game). Nor, in PnP, is there upwards a dozen or more 'encounters' per dungeon.
NWN is a video game. It has to use video game mechanics. Everything moves at a much faster pace. There's a lot more monsters per dungeon. There are larger spawns of things that can completely decimate certain classes, because NWN was made with the ability to solo in mind as well. Doubt anyone at Bioware expected NWN's multiplayer to be as popular as it became, that it's still being played so avidly long after they stopped supporting it.
So with keeping an ability to hold your own against most spawns, different ways were programmed in to give classes that big weaknesses the ability to handle most situations, Knockdown being one of them.
Now then, let's take a look at PotM.
There's a lot more risk in death (Sorry, in PnP the odds that someone is going to nick your weapons and money are -extremely- low, because again...COOPERATIVE. That, and it's monitored heavily by a DM and 'most' DM's are pretty lenient with death in a PnP game.). There's a higher chance for PvP. There's corpse hiding, removing someone from play for extended periods of time and basically denying another player the right to play their PC because your PC doesn't like them. Despite the rules encouraging cooperation with PvP instead of competition, I'm sure at least 80% of the people that involve themselves in PvP care a hell of a lot more about their own PC's than they do the ones trying to kill them, and will act accordingly. (Personally, I think the 'reasons' most people PvP for is insane. Murder and violence seems to be the only action taken against anyone ever for every reason. 'You took my sweet roll! *Stabs in the face*'.)
All in all, comparing NWN to PnP is apples and oranges. Two completely different things, but they're both fruit. When I boot up my launcher, I don't see 'Dungeons and Dragons' anywhere on it. The only thing I see even hinting that it might be like D&D is the Wizards logo. NWN is not PnP. It is a video game based on PnP roots.
Knockdown can be countered in various ways. Timestop, Missle Storm, ect, cannot. Sneaks can also be countered, though that seems to be the easiest done by casters.
Conclusion? Casters caught unaware are the easiest to kill. But when one is ready and expecting a fight, they'll win. Always.