Open Source.

Transmission is an open source, volunteer-based project. Unlike some BitTorrent clients, Transmission doesn't play games with its users to make money:

Transmission doesn't bundle toolbars, pop-up ads, flash ads, twitter tools, or anything else.
It doesn't hold some features back for a payware version.
Its source code is available for anyone to review.
We don't track our users, and our website and forums have no third-party ads or analytics.

Easy.

Transmission is designed for easy, powerful use. We've set the defaults to "Just Work" and it only takes a few clicks to configure advanced features like watch directories, bad peer blocklists, and the web interface. When Ubuntu chose Transmission as its default BitTorrent client, one of the most-cited reasons was its easy learning curve.
Lean.

In separate benchmarks, Linux Format and Lacrocivious both found Transmission to use less CPU than any other GUI client. It even used less CPU than some non-GUI clients.

Transmission also has the lowest memory footprint of any major BitTorrent client.

Imageshack chose Transmission for its BitTorrent farms because the competition "requires amounts of memory several times greater than Transmission".

https://transmissionbt.com/about/