Torrent Limitation

Content unavailability

Although swarming scales well to tolerate flash crowds for popular content, it is less useful for unpopular content. Peers arriving after the initial rush might find the content unavailable and need to wait for the arrival of a seed in order to complete their downloads. The seed arrival, in turn, may take long to happen, since maintaining seeds for unpopular content entails high bandwidth and administrative costs, which runs counter to the goals of publishers that value BitTorrent as a cheap alternative to a client-server approach. A strategy adopted by many publishers which significantly increases availability of unpopular content consists of bundling multiple files in a single swarm.

Lack of anonymity

BitTorrent does not offer its users anonymity. It is possible to obtain the IP addresses of all current, and possibly previous, participants in a swarm from the tracker. This may expose users with insecure systems to attacks.^[9] It may also expose users to the risk of being sued, if they are distributing files without permission from the copyright holder(s). However, there are ways to promote anonymity; for example, theOneSwarm project layers privacy-preserving sharing mechanisms on top of the original BitTorrent protocol.

The leech problem

A BitTorrent user may often choose to leave the swarm as soon as they have a complete copy of the file they are downloading, freeing up their outbound bandwidth for other uses. If enough users follow this pattern, torrent swarms gradually die out, meaning a lower possibility of obtaining older torrents (see content unavailability above). Some BitTorrent websites have attempted to address this by recording each user's download and upload ratio for all or just the user to see, as well as the provision of access to newer torrent files to people with better ratios. Users who have low upload ratios may see slower download speeds until they upload more. This prevents (statistical) leeching, since after a while they become unable to download at even a fraction of the theoretical bandwidth of their connection. Some trackers exempt dial-up users from this policy, because their uploading capabilities are limited. The BitTorrent protocol also attempts to minimize the damages of leeches by using only a portion of their bandwidth for one-directional trades and using the majority for two-directional trades that tend to help the swarm as a whole.

The cheater problem

There are "cheating" clients like BitThief which claim to be able to download without uploading. Such exploitation negatively affects the cooperative nature of the BitTorrent protocol, although it might prove useful for people in countries where uploading copyrighted material is illegal, but downloading is not.

Speed

Average BitTorrent download speed is usually the sum of that peer's upload speed and a fair share of the total upload of all the "seeders in the swarm" (peers logged with the tracker that have a complete copy of the file). The 'tit-for-tat' style file sharing of downloading peers is responsible for the portion of the available download that's the same as the peer's upload. The seeders attempt to provide fair shares by scattering pieces to a wide selection of the best performing peers.
ISPs often provide asymmetrical Internet connections, with much higher download than upload speeds. Since a peer can only download data that's been uploaded by another peer this asymmetry is suboptimal for the bittorrent protocol. This performance issue is most obvious during the early life of a swarm when there is only one peer that is seeding (has a complete copy of the file) and all the other peers have exactly the same portion of the file as each other. When you initially join such a swarm you will get very high download speeds as every other peer optimistically sends you pieces in the hope that you have something to send them. This will probably continue until the time your peer catches up with the rest of the swarm when your average download speed drops to exactly the same as the upload speed of the seeder. The data is uploaded by the seeder to one peer and that peer passes it on down the line to the next in the swarm and so forth to everyone in the swarm.
If all peers in the swarm have symmetrical connections the swarm becomes far more stable. During the initial startup the swarm will be less able to draw new arrivals to the current maximum level of the swarm so the "everybody becomes a seeder" threshold is much less of an instant switch and more of a controlled cascade. The balance between the upload and the download also means that the majority of a peer's download is as a result of the 'tit-for-tat' file sharing which reduces the cost of seeding to a swarm by forcing the natural ratio of a peer closer to the overall 1:1 requirement of the swarm as a whole. If this reduction in 'seeder cost' were to happen in the wild it would probably result in much longer lived swarms too.
As symmetrical connections are uncommon swarms are normally in the "seeder starved" state where there is very little seeder upload bandwidth available and each peer gets about the same download as it's upload. Additional upload bandwidth can be made available to a swarm through the use of "seed boxes" and "http seeds" but this is quite rare with public torrents.
Source : Wikipedia