You write:

"If a cook wanted to sell his videos online, what advantages would a cooking themed video selling site have that a generic video selling site not provide?"

This question only makes sense if you reject my thesis that this project will mostly be about marketing (not technology). If you accept that this project is going to be mostly about marketing, then the question answers itself - you take a niche approach because it makes the marketing easier.

The statement "makes the marketing easier" rests on 2 assumptions:

1.) you will have on your team someone from the niche that you are targeting

2.) you are willing to share revenue so as to avoid the expense of marketing

Again, if you have $10 million to spend on marketing, you could take a brute-force approach and try to do a generic site and just market the hell out of it. But if your marketing budget is $0, then lining up partners who are in the niche that you are targeting is a wise move.

Above (in another comment) I listed 4 attributes I'd look for in the partner who is in the niche that I'm going after. The important thing is that they have some videos to sell, and the ambition to want to go out and make the site a monster hit. That way you can leave a lot of the marketing work to them. Again, my reference here is what Tilak Pyle was able to do for iHanuman. His video was the best selling thing on iHanuman, and its sales were an important part of what made that site seem legitimate to the other yoga teachers who eventually joined the site.

You write:

"just changing the theme of the site."

I think that is an astonishing phrase. You are seriously discounting the importance of marketing. You are seriously discounting branding and the value of good graphic design.

You wrote:

"my main focus right now is a niche: independent films"

If I were to try this again, I'd go after consumer niches, not producer niches. I mean, the niche should be defined by the consumer group, not the producer group. The niches I'm thinking of are the consumer niches: cooking, hunting, racing, gambling, yoga, health, stock market trading, etc.

You wrote:

"but that's assuming I'm trying to build a marketplace"

Sorry, I must have misunderstood you. What are you building then? I can not imagine how you would bring distribution to indie film makers without building a marketplace. If you are not selling then what are you doing for them?

You wrote:

"And even then, categorizing videos should suffice."

You may have better luck than what I've seen in the past. I certainly wish you luck. I can tell you, with great certainty, that the enthusiasm that the yoga teachers showed for iHanuman would not have been there had we simply had a generic sell-videos site that just happened to have a "yoga" category. The marketing strategy that worked for iHanuman was "We belong to your community. We are one of you." After all, doesn't Amazon have a digital video sales channel at this point? The yoga teachers could just use that if they wanted some soulless, generic service.

You wrote:

"I also think focusing on any of the niches you suggested would just narrow the potential customer base (considering content producers as customers), even though it may make stuff easier to find for end users."

It seems to me that this is the essence of your business plan, and it is the crux of where you and I see things differently. What you describe is exactly what I think will fail. However, I certainly wish you luck. I've known a lot of good folks who tried to launch similar services and I know it is a tough market to crack. If you can make this work, then you'll have succeeded at a terrific challenge. I do not think it will work, but I wish you well.