Since version 2.00, Transmission has Local Peer Discovery feature, which seems quite useful. I was surprised that it wasn't enabled by default though, so I became concerned about any potential issues it might have.
As far as I could investigate there are not major issues with the LPD feature, but maybe the reason on this issue tracker ticket.
Check this comment from project issue tracker:
There needs to be a manual toggle to enable/disable the feature globally, and it MUST default to disabled. Please let me justify this last requirement. The local peer discovery code broadcasts the set of torrents you're participating in every four minutes. Anyone on the same link as you (your neighbour, your mom, your wee brother) can find out the set of torrents just by listening on the right multicast group, and these are the very people you might not want to know about what your are downloading. Hence, I insist on this being DISABLED by default.