I think one of the main reasons the NWN1 & 2 engines have lasted so long is that there simply isn't anything else to tell a D&D story with. Sure, you can hop over to Dragon Age and build in that engine, but you're limited to their few classes, spells and lore. I think most of us have D&D in our DNA because it goes so far back into our childhoods with the PnP game. Having an engine available to build in is priceless. As long as D&D continues to put out books and have a PnP presence, people are going to look to the computer for supplemental play opportunities, and they're going to find the NWN games and mods. And since Atari/Bioware seems to have no inclination to put out a 3rd, non-online version of the engine, this is the only game in town. Hence, I think it will continue to have life and there will be people who will play our stuff.
Also, take a look at the download numbers for the Baldur's Gate Reloaded mod, and then know they're working on BG2. I think if people leave NWN2 alone for a while, they're still going to leave it installed, and they're going to download the BG2 campaign when it is released. I plan on having my work ready to go around the same time 
Anyway, long story short: my opinion is that the game will continue to have life for as long as it can run on a modern CPU, because it is the only computer D&D engine around.
As to the questions:
1) I think Tchos made the best point here. Personally I don't like small spaces very well, but he brings up a great point when he talks about choke-points. That is a tactical element that is totally valid. It does make the player rethink certain things (like in TBCOCC, Tcho's mod, inside the lighthouse - I learned to have my rogue unlock the door and then back away and use a ranged weapon, and let the heavies actually open the door).
2) I echo Tchos and RJ here. I prefer having a big party. I like having one of every archtype, and then I like having room for flavor character as well. My campaign is allowing 6 total (you and five companions). I liked that party size in the original Baldur's Gate games and the Icewind Dale series. It just seems "right" IMO. And as a player who likes tactical combat, I like having options.
3) Yes. A game can try and do too much. I think the default NWN2 engine is nicely balanced though.
4) I don't play on a huge monitor with a giant resolution (yet), so I don't have an issue with the interface. I will say that I like the consistent look and feel of the default engine's interface (colors and styles). The only thing that would be nicer is some more information, which I can get through Charlie's UI (mainly Spell Resistance number on the main character interface).
As to what module works best: I don't think there is a specific. I think the engine is flexible enough to allow several different kinds of modules, and think some authors have figured out ways to best create those types of modules. Big or small, focused on a small set of companions or a single companion, a big story or an intimate one, what matters is that the person creating the mod tells that story well and does a good job implementing their vision.
What I enjoy most about the game is tactical combat situations, combat in general, well-designed and interesting-looking areas, and good writing.
For instance, I just finished Harp and Chrysanthemum a couple months ago, and while I was irked/disappointed by certain elements, overall I thought it had some of the strongest writing in any mod I'd played. The dialog jumped out at me as being professional quality, and I really liked that. I like good writing: characters that say dialog that matches their alignment and motivations, and doesn't just exist to push the plot forward (or worse, pushes the plot forward while at the same time being something that character would NEVER say due to their alignment or personality).
I like well-balanced combat. I don't like it when combat situations are too difficult. I don't play this game to be like an RTS on the highest difficulty setting. I enjoy a challenge, but I don't want to reload an encounter 10 times. Reloading a couple times to try different tactics is fine. But being forced to reload multiple times in the hopes that the RNG will simply be nice and swing the battle in my favor is lame.
I like great area design. Tchos should teach a class. I loved wandering around his mod The Black Scourge of Candle Cove (which I have not finished yet, but will). His areas are beautiful and should be a lesson for everyone who is building. I know I've stolen a lot of ideas from his mod.
My overriding though on this whole thread is this: the game engine is fine the way it is. It's a decade old now and we're still building with it. People like Tchos and other modders have proven we can build really great looking areas with it, and the community just keeps adding content; we're seeing new placeables and monsters and effects all the time. Everything else is on the builder to simply and effectively tell a great story and give us fun encounters.
The engine doesn't need any tweaking. What we need is more good stories. Fortunately, I think there are several in the pipeline, and there's a long list of mods I personally haven't played yet...