(SVG Long Tail Diagram by Hay Kranen via Wikipedia)
BitTorrent is a highly useful technology that helps to share the burden of many people getting access to content available from the internet by distributing the load via a P2P network.
The big problem with bittorrent is that it's not great at providing access the Long Tail of content. Before we continue here's a very brief overview of the Long Tail. If you've not had the chance to read Chris Anderson's book - The Long Tail - I'd highly recommend it.
The definition of the Long Tail comes from a measurement of popularity as shown in the diagram above. The left hand side show the items that are most popular - think chart hits if you're talking in terms of music. The right-hand part of the curve in the diagram above is the Long Tail. The Long Tail represents lots of niche choices (Any obscure or rare album would fit if in our musical example).
The reason why the Long Tail is important is that millions and millions of individual sales of products from the Long Tail adds up to a lot of money. Even if each item's own popularity is minimal. The Internet makes the Long Tail market more of a success due to the fact of virtually infinite shelf space. Again sticking to music as an example if you're offering downloads then you can afford to offer lots of niche albums as the cost of having them available is negligible. Where physical, finite shelf space is concerned you are only going to find more popular titles as shops need to stock what they know they can sell.
Bittorrent is not well suited to surfacing Long Tail content because Bittorrenting relies on there being someone who is seeding the data. For head content such this is not a problem - as popularity ensures the availability. However, if you're trying to track down niche content - you might well find a torrent but you'll find there's no-one seeding the content in which case you lose.
BitTorrent cannot provide as much choice available to other means of distribution which is kind of strange as you often think of Bittorrent as providing endless choice at your fingertips - if you have mainstream tastes then it will work for you, if you don't your satisfaction might be more limited.
Bittorrent is great for popular content where demand is high. It's distributed approach makes it possible for large chunks of content to be made available to lots of people at once. As such for fresh and popular content it can be the perfect means of distribution but by virtue of how it works it will always cater more towards the head content.