Let me throw in my 2 cents.
I've worked with SharePoint WSS (1.x, so my arguments for/against it are dated and I'll just leave them out) and I've worked with DNN.
I agree that WSS/SPS is absolutely great when it comes to all the document management, collaboration, etc, especially with it's 100% integration with ALL Office Products (infopath, outlook, etc). We had a Microsoft rep out to our company the other day trying to get us to use it because our corporate licensing actually already provides it for us. Turns out that v2.x (the latest variety) worked out all of the hangups that 1.x had. I used to hate WSS with a passion!
As mentioned previously, there is no reason why DNN can't have those capabilities either. The problem is that nobody has really sat down and created a module to do it that good. WSS/SPS are great because they have it "out of the box". DNN is great because it is generally free (aside from the large commercial module base), but even those items that aren't free are very affordable. Case in point, our previous intranet provider recently recently a forms module for their portal with an "introductory price" of $3k. DNN has SEVERAL forms providers out there ranging from $30 to $200, but they are pretty freaking nice (IMHO, the higher price ones are nicer... the lower cost ones tend to be lacking).
DNN core, of course as mentioned, is VB... I am a C# developer personally and have developed in excess of 15 modules for DNN--they plug in nicely and play nicely, lots of good articles about that, but I suggest you use the WAP model for integrating them.
Lastly, have you ever actually tried to use Microsoft support for anything? They suck. The DNN community is vast and although many questions go unanswered, at least they try. I've had to call Microsoft support on various occasions because my superiors asked me to "give it a try" and every single time I came back with less brain cells than I started. In every case I ended up solving the problem on my own using some nice Googling because their techs were too dang dense or their english was so freaking bad either I couldn't understand them or they couldn't understand me. My experience with DNN "support" has been much better.
Am I saying that I'd prefer one over the other? Sure, in this current place in time, I prefer DNN. Am I saying that I'll never consider SPS/WSS again? No, not particularly. If DNN can't deliver a document management system to compare with what WSS/SPS has, I may see myself moving to it in the next couple of years as the older employees retire and we replace them with younger blood whose minds could comprehend that sort of thing (no joke).
WSS is possible to pull a website out of though... check out some I did with my previous job:
http://www.mympa.org/default.aspx
http://www.cohousing.org
That was with the crappy old 1.x variety too. Pain in the buttholio though.